The bedded or lenticular deposits of manganese oxide ore in Miocene sedimentary rocks are distributed in the so-called green tuff region, especially in the Southwestern part of Hokkaido and the Northeastern half of Honshu. The ore deposits occur in the formations, which are considered correlative to the Daijima, Nishi-kurosawa and Onnagawa in the middle Miocene. The formations containing the ore deposits are, in many cases, of shallow sea deposits, and characterized by the rather thiner strata in thickness or the local unconformities in the basal part. Based on the abovementioned facts, it seemed that the paleogeographic conditions, in which the ore deposits have been formed, were of shallow sea, near the marginal places of sedimentary basin.