The 2004 Kushiro-oki earthquake generated a small tsunami that was observed at two tide gauge stations located on the Pacific coast of Hokkaido Province. Analysis of the tsunami waveforms shows that the slip amount of the fault was 2.1 m. The seismic moment was calculated to be 3.1×1019 Nm, which is consistent with the results of previous seismological studies. Tsunami simulation results indicate that a small first wave at Urakawa is caused by large shallow water off Cape Erimo. The tsunami generated from the source area off Kushiro circumvents the shallow area off the cape, propagates through the deep sea, and arrives at Urakawa as a small first wave. Larger tsunamis are propagated through the shallow region slowly and arrive at Urakawa as a later tsunami. These results suggest that a tsunami from a future large Nemuro-oki earthquake will also arrive at the west coast of Hidaka with a small first wave and large later phases.