This paper describes the eruption products and structure of Katsuma-Yama volcano. Katsuma-Yama volcano is located in the northern part of Okushiri Island off the Oshima Peninsula, Hokkaido and has three eruption centers: Horonai-Gawa caldera, Katsuma-Yama crater and west Katsuma-Yama crater. Horonai-Gawa caldera is 2km by 1.5km in dimension, and is filled mainly with massive breccias to tuff breccias, fine-grained lacustrine sediments and bedded lapillistone to tuff. Massive breccias to tuff breccias are exposed along the western wall of the caldera and have been recovered by drilling in the central portions of the caldera. The breccias contain fragments mainly of dacite, dacite pyroclastic rocks, andesite, and granodiorite together with minor fragments of perlitic rhyolite. The majority of the rock fragments are quite similar in their constituents and textures to the surrounding basement rocks. Perlitic rhyolite, however, is relatively fresh and cannot be recognized in the surrounding basement rocks. This rock is, therefore, thought to be juvenile, although no eruption products remain outside the caldera. The fine-grained lacustrine sediments are wavy stratified with a wavelength up to a few meters and locally contain sulfur deposits. Bedded lapillistone to tuff comprises mostly fragments of glassy biotite-rhyolite, and projectiles derived from the direction of Mt. Katsuma Yama formed sag structures in the beds. Katsuma-Yama crater occurs at Mt. Katsuma Yama. The crater has a diameter of 740m across and is filled with bedded lapillistone to tuff. The constituents are mostly non- to poorly vesicular polyhedral or platy fragments of glassy biotite-rhyolite and are thought to be phreatomagmatic in origin. Although the outflow deposits partly remain on the western and southeastern flanks of Mt. Katsuma Yama, most of the expected pyroclastic ring or cone has been removed through later erosion. Katsuma-Yama lava of similar composition occurs through the crater infill, spreads over the eroded surface of the outflow deposits, and is distributed mainly on the southwestern flank of Mt. Katsuma Yama and further to Horonai-Gawa caldera. The lava has a thickness of 100m at Mt. Katsuma Yama, thins to the downflow directions with a variable thickness of flow breccias, and is intruded into the infill of Horonai-Gawa caldera, with plastic deformation of the caldera deposits along the contact. West Katsuma-Yama crater opens through Katsuma-Yama lava at its western margin, with a diameter of 180m. The major infillings comprise rock fragments mostly similar to those from the Katsuma-Yama crater. A minor volume of pyroclastic surge deposits from this crater remains on Katsuma-Yama lava in the summit area of Mt. Katsuma Yama, and rests on the brown soil of a few centimeters that covers the eroded surface of the western rim of Katsuma-Yama crater. The eruption volume from Horonai-Gawa caldera is unknown but could be between 1 and 10km³. The eruption volume from Katsuma-Yama crater perhaps slightly exceeds 0.6km³, and the eruption volume from west Katsuma-Yama crater is very small, perhaps less than 0.01km³. Katsuma-Yama lava is dated by fission-track methods to be 0.2-0.7Ma, and no soil occurs between the lava and the overlying pyroclastic surge deposits of the west Katsuma-Yama crater origin. A thin brown soil between the pyroclastc deposits of Katsuma-Yama and west Katsuma-Yama craters represents a short dormancy in volcanic activity. Lacustrine deposits in Horonai-Gawa caldera indicate a high wave-energy setting. Sulfur precipitated in the deposits suggests fumarolic activity in the caldera lake. These facts likely demonstrate post-caldera volcanism, and the plastic deformation of the caldera fill by the intrusion of Kamui-Yama lava suggests that the post-caldera volcanism was succeeded by the activities of Katsuma-Yama and west Katsuma-Yama craters.