Mount Usu is a Quaternary composite volcano located in southwestern Hokkaido, Japan. Here we report on an andesitic pyroclastic fall deposit (the Usu-Kaminagawa [Us-Ka] tephra) erupted during the initial stage of activity at Usu Volcano. The tephra extends from the volcano to the east, and comprises a lower andesitic pumice-fall deposit and an upper ash-fall deposit. The tephra overlies the Nj-Os tephra, which was erupted from the Nakajima Islands, and is overlain by the Usu Somma Lava, which was extruded during the early stages of activity at Usu Volcano. Radiocarbon dating of buried soils located immediately beneath the Us-Ka tephra yields ages of 18-19 cal ka BP. The distribution, stratigraphy, and lithology of the tephra, and the radiocarbon ages of the buried soils beneath the tephra, suggest that an andesitic explosive eruption occurred at Usu Volcano at ca. 18-19 ka. This eruption was probably an early manifestation of activity at Usu Volcano.