The Toyoha mine situated at about 30km southwest of Sapporo city, is working on epithermal vein systems which occur along some shearing and tensile cracks in green tuff, propylite and black shale of the Miocene series. The ore veins carry abundantly galena, sphalerite, pyrite and quartz and lesser amount of rhodochrosite. A little hematite and magnetite are detected in the lower part of these veins. In this paper, genesis of the iron oxides in the epithermal vein is discussed.