Lake Kinrin, a small and shallow lake in Yufuin Town, Oita Prefecture receives two kinds of groundwater inflow. One is high-temperature water of 32-35℃ from a hot spring and the other is relatively low-temperature water of 15-20℃ from springs in the bottom and around the periphery of the lake. The diatom flora in the lake influenced by high-temperature water was compared with that of two sites lacking such influence. In total, 147 diatom taxa belonging to 46 genera were recorded. At the site influenced by the hot spring, Achnanthes exigua and Nitzschia amphibia were dominant, and Pinnularia joculata was a characteristic species in the community. At the other sites, Nitzschia fonticola, Fragwasilaria neoproducta, Planothidium lanceolatum, Martyana martyi, and Staurosira elliptica were the dominant species.