The Jokoku mine, the largest manganese mine in Japan, is in the south western part of Osima peninsula, Hokkaido. The ore deposits are of epithermal veins of Miocene age, consisting chiefly of rhodochrosite with minor amounts of sphalerite and galena. The Fe content of the rhodochrosite is from 4 to 10 percents. The rhodochrosite formed in the earlier stage of the mineralization is relatively rich in Fe and that in the later stage in Ca and Mg. Champion vein occurs in a wide shear zone which forms the boundary between Palaeozoic slate and Tertiary andesitic breccia. Several veins have been found branching out into the andesitic breccia.