Oshima Island & the Geiyo Fortress Ruins: Imabari’s Hidden History by Ferry
Just off the coast of Imabari, in the swirling currents of the Kurushima Strait, sits a tiny island most travelers never notice. It has no shops, almost no residents, and can only be reached by a small local ferry. Yet Oshima Island (小島, literally “small island”) hides one of the most remarkable sights in the area: an early-1900s coastal fortress, left almost exactly as time and nature have shaped it.
For history lovers and curious cyclists riding the Shimanami Kaido, it makes a memorable half-day detour. Admission is free, the island is open 24 hours, and a relaxed walking loop of the ruins takes about two hours.
A quick note on the name: this small Oshima (小島) is not the same as the larger Oshima (大島) on the main Shimanami cycling route. They just share a similar-sounding name.
A fortress frozen in time
Oshima is part of the Setonaikai (Seto Inland Sea) National Park. In the late Meiji era, around the turn of the 20th century, the Geiyo Fortress was built here as a coastal artillery position, part of Japan’s defenses in preparation for the Russo-Japanese War. Its guns were meant to watch over the Kurushima Strait, one of the key sea lanes into the Inland Sea. The fortress was abandoned only about twenty years after it was completed, and the island has kept its ruins ever since.
What makes Oshima special is how much survives. Gun batteries, ammunition depots, brick barracks, underground rooms, a searchlight station and command towers still stand across the island. There is no polished museum here and no crowds, only occasional volunteers keeping the paths clear, so you walk through the ruins much as they were left, wrapped in quiet forest. In spring, a promenade lined with some 2,500 camellia trees adds a splash of color.
Why Oshima is worth the trip
If you are deciding whether to make the crossing, here is what makes Oshima stand out.
Most fortress sites in Japan are tidied up and busy. Oshima is the opposite: you often have the whole island almost to yourself, and the ruins are left raw and unrestored, so exploring them feels like a genuine discovery rather than a museum visit. It is free and open around the clock, the walking loop takes only about two hours, and it slots easily into a Shimanami cycling trip or a day around Imabari. The brick barracks, gun batteries and bridge views are wonderfully photogenic, and children with an adventurous streak tend to love it. It is also one of the quieter places to see the Kurushima Kaikyo Bridge, away from the usual viewpoints.
Getting there: the ferry from Hashihama Port
The island is reached only by ferry from Hashihama Port, and the ferry is the only way both there and back.
From JR Imabari Station, take the Setouchi Bus “Namikata Loop Line” for about 20 minutes and get off at Watashiba (渡し場). Hashihama Port is a 1-minute walk away. From there, the ferry to Oshima takes about 10 minutes.
Hashihama is a shipbuilding town, so as you wait you will see huge shipyard cranes lining the water, with the Kurushima Kaikyo Bridge beyond, a memorable view in itself. The ferry runs to Kurushima, Oshima and Umajima in that order. There are about nine sailings a day, roughly from 7:00 to just after 17:00. The one-way fare is about 220 yen for an adult, plus 90 yen if you bring a bicycle.
Hashihama Port is a small, unstaffed port, so buy your ticket from the vending machine before boarding the little ferry that carries you across.
You can check the location of the port here: Hashihama Port on Google Maps.
Walking the ruins
As the ferry pulls in, you already spot a replica cannon standing above the little stone pier, a hint of what is waiting.
At the landing, a single small signboard marks the start of the trail. Check the promenade map here: from this point it is essentially one path, roughly 1.8 km in total, though a few branches and mountain steps appear along the way.
Near the start is the island’s only rest area, with a replica cannon, benches, a vending machine and the sole toilet, watched over by a few friendly stray cats. It is worth pausing here before you head off.
The trail climbs first to the searchlight station ruins, reached by a stone stairway that disappears into the trees.
You can step right inside the small stone rooms, low-ceilinged but larger than they look, with further chambers opening up behind.
From here the path continues to a power station and underground rooms, many of which you can also enter. Some still hold traces of their old fittings, slowly filling with fallen leaves.
The ammunition depot has lost its roof, but as the structure built to store explosives it is the sturdiest of all, with thick walls and foundations still clearly laid out.
One of the most atmospheric spots is the old brick barracks, its roofless walls and rows of floor foundations slowly being reclaimed by grass and trees, a little like a scene from a Ghibli film.
The highlight is the Central Battery, the best-preserved section, where six gun emplacements were built into hollows in the hillside so they could not be seen from the sea. Rows of barracks stand just behind it.
The command-tower lookout
Behind the battery, a steep staircase leads up to the command-tower platform.
At the top you are rewarded with the view that many visitors remember most: the wide blue strait, scattered islands, and the great white Kurushima Kaikyo Bridge stretching across the horizon. There are benches here, making it the perfect place to catch your breath and take it all in.
The stairway down is genuinely steep, so take it slowly.
The northern battery
Beyond the central section lies the Northern Battery, with four gun positions in two groups. The pedestals that once held the guns are still in place.
One of them still shows marks from old bombing practice, with a dramatically broken gun port. A second, smaller command tower nearby gives another good sea view.
The walk back
From here the trail loops down to a quiet coastal road that follows the shoreline back to the port, with the bridge in view for much of the way.
Before you go: a few practical tips
Oshima is beautifully undeveloped, which is exactly why a little preparation helps.
There is just one rest area on the island, with the only vending machine and toilet, so bring your own water and snacks. Wear proper shoes: the trail includes mountain steps and some stairs where the surface has crumbled. In the warmer months you may see hornet-warning signs, so stay on the path.
One thing to take seriously: Oshima is almost deserted. Only a handful of people still live here — so few that within a few years the island may have no residents at all — and you can walk the whole trail without meeting anyone. There is no shop, no staff and nowhere to stay. A few houses remain near the port, so in a genuine emergency you might be able to knock and ask for help, but you should not count on it. That solitude is part of the magic, yet it also means the ferry is your lifeline. Decide your return boat before you even set off, and be back at the pier with time to spare. Miss a daytime boat and the next one is an hour or two away; miss the last sailing (around 17:10) and you could be stranded on the island. Always check the current timetable, and please carry your rubbish back with you.
A suggested half-day plan
Because everything hinges on the ferry, it helps to plan around the timetable. A simple approach is to decide your return boat before you cross, take a late-morning boat over, spend about two hours walking the ruins, and catch an early-afternoon boat back. As a rough example based on the current schedule, the 10:10 sailing over and the 12:45 or 14:00 sailing back fits nicely, but the exact times change, so always check the latest timetable before you go. Allow roughly half a day in total: about 10 minutes each way on the water, plus around two hours on the island.
Good to know
Is it suitable for children? Yes. Kids often love exploring the batteries and the underground rooms, and there are stray cats to spot along the way. Just keep an eye on them, as there are steep steps and some crumbling stairs.
Is there anywhere to eat? No. There are no shops or restaurants on the island, only a single vending machine at the rest area, so bring your own food and drink.
What about rain? The ruins are all outdoors and the stone steps get slippery when wet, so a dry, clear day is much more comfortable and safer.
Are there English signs? Signage is mostly in Japanese, with a little English here and there. The main trail is essentially one loop, though, so it is hard to get lost.
Are there toilets? There is one toilet, at the island’s only rest area near the pier. There are none further along the trail, so it is worth stopping there first.
What if I miss the last ferry? Take this seriously: only a handful of residents remain and there is no lodging, shop or staff, so you may not meet anyone who can help. There are a few houses near the port where you could try knocking in a real emergency, but do not rely on it. Aim to be back at the pier well before the last sailing (around 17:10), and always confirm the current timetable before you cross.
A great detour for Shimanami cyclists
Oshima is actually the island you see to the left when you look out over the strait from Itoyama Park, and the same ferry stops at Umajima, which sits right under the Kurushima Kaikyo Bridge on the cycling route. If you are exploring the area by bike, it pairs naturally with a Shimanami ride. New to renting here? See our guide to renting a bicycle in Imabari, and after a day of walking and pedaling, a soak at Kisuke no Yu hot spring near the station is hard to beat.
Few places let you wander through a century-old fortress in near silence, with the sea on every side. If you like history, quiet nature and a touch of adventure, Oshima is well worth the short crossing.
