Mount Apoi, Mountain summit in Samani, Japan

Mount Apoi is an 810.5 m summit at the southwestern end of a spur of the Hidaka Mountains in Samani, Hokkaido, with a first-order triangulation point named Fuyushima. It is built of peridotite known as Horoman peridotite, whose thin, dry, nutrient-poor soil keeps forest from developing, so alpine plants grow at low altitude. Above the fifth station the forest limit is reached, yet a dwarf birch stand above the ninth station blocks the summit view.

Mount Apoi is built of ultramafic rock, including the peridotite known as Horoman peridotite, of unknown age. Its alpine plant community was designated a Special Natural Monument in 1952, and in 1981 the mountain became a special protection zone of the Hidaka Sanmyaku-Erimo Quasi-National Park. The surrounding Samani area was certified as a Japanese Geopark in 2008 and a UNESCO Global Geopark in 2015.

The name Apoi probably comes from the Ainu word ape-o-i, meaning "place where fire burns", though its old meaning is uncertain. A legend tells that a deer-hunting ritual with a fire altar on the mountain brought abundant deer. Today local volunteers patrol the slopes to stop poaching of Hidaka-sō and other endemic plants.

The trailhead is reached from Samani town; the Apoi岳 Geopark Visitor Center, opened on 13 April 2013, provides orientation and trail information. The route crosses peridotite slopes where thin, dry soil and strong winds make footing loose, and summer sea fog can lower temperatures. The forest limit is around the 5th station, but dwarf birch stands above the 9th station block the summit view.

The Himechamadara Seseri, a brown butterfly with small white speckles about 2.5 cm long, lives only on Mount Apoi and nowhere else in Japan; its larva feeds on the Kinrobai flower and adults fly only in May and June. The Apoi Maimai, a snail recorded as a new species in 1970, hides in crevices of the peridotite rock.