Mt. Io (Iozan): Walking Among the Steaming Vents Near Kawayu Onsen

Mt. Io, or Iozan, is an active volcano at Kawayu Onsen in eastern Hokkaido, and you can walk right up to the vents where steam roars out of yellow, sulfur-crusted rock. The Ainu name is Atosanupuri, "the naked mountain", because almost nothing grows on its slopes. Walking the path is free (parking is 500 yen in summer), there is a rest house with a shop, and it fits easily between Lake Kussharo and Lake Mashu.

What is Mt. Io like?

The Mt. Io sign of Akan Mashu National Park, with steam rising from the volcano behind

Mt. Io is an active volcano in Teshikaga, inside Akan Mashu National Park. The bare rock is streaked with yellow sulfur, white steam pours out of the ground, and sulfur was once mined here.

The smell of sulfur hits you as soon as you get out of the car. As you walk along the path, you hear the vents roaring and the steam comes right up close. It was far more powerful than in photos, and it really felt like standing next to a living volcano.

Steam pouring from vents crusted with yellow sulfur at Mt. Io

Unlike the viewpoints over the lakes, you do not just look at Mt. Io: you smell it, hear it and feel the heat. The strength of the smell and the look of the steam change with the wind, even in a short visit, and it makes a sharp contrast with the calm of Lake Kussharo nearby.

A sulfur-crusted vent with steam at Mt. Io

Iozan MOKMOK Base

Right by the volcano is Iozan MOKMOK Base, a new, clean rest house with toilets and a place to sit. The shop sells eggs boiled in the volcano's heat, soft-serve ice cream and drinks, and it was good to have somewhere to take a break after the walk.

Hours: 8:30–17:00. It closes on some days without a fixed schedule, and may close or shorten hours in bad weather, so check before you go
Parking: 500 yen for a car in the summer season, free in winter. The same ticket covers one visit to the car park at Lake Mashu Observatory No. 1 (the main viewpoint over the lake), on the same day only.

How long to spend at Mt. Io

About 30 to 40 minutes is enough to walk the path and take a break at MOKMOK Base. Even on busy days the car park turns over quickly.

Things to know before you go

The smell of sulfur is very strong. If it bothers you, keep the visit short.
Stay on the path. The bare ground can be very hot.
On windy days it feels cold, so bring a layer.

Near Mt. Io

Mt. Io is about five minutes by car from Kawayu Onsen, the hot spring town with strong, acidic sulfur water, and close to Sunayu on Lake Kussharo, where you can dig your own foot bath in the sand.