Mt. Shiretoko-iozan is a Quaternary andesitic composite volcano located on the Shiretoko Peninsula, eastern Hokkaido, Japan. An amphitheatre at the summit and a debris-avalanche deposit (Minamidake Debris-Avalanche Deposit) at the western foot of the volcano suggest that sector collapse occurred during its eruptive history. Three trenches (6.8, 7.3, and 4.2m deep) were dug at the western terminus of the Minamidake Debris-Avalanche Deposit in order to date the sector collapse event. The stratigraphic section consists of (from lower to upper): the Sashiruidake Lower Lava, a debris-flow deposit, a buried soil, a debris-flow deposit, a buried soil, the Minamidake Debris-Avalanche Deposit, a buried soil, the Ta-c tephra, and the surface soil. Radiocarbon dating of a buried soil located immediately beneath the Minamidake Debris-Avalanche Deposit yields an age of 3740±40 years BP (4230-4190 and 4190-3980 years cal BP), suggesting that the sector collapse occurred at ca. 4ka.