Plate boundary slip associated with the 2003 Off-Tokachi earthquake, northern Japan is determined by analysis of repeating earthquakes in the source region. A total of 251 repeating earthquake groups consisting of 666 events are identified in and around the aftershock region from October 1, 2000 to May 31, 2004. Although a number of the repeating earthquakes in this region occurred before the main shock, there are many events that occurred after the main event. The repeating earthquakes are located in the major afterslip region and distributed to the southeast. Based on this analysis, it is estimated that the areas to the east and southeast of the coseismic area undergone approximately 50 cm of slip over three months following the earthquake, and less than 5 cm over the three years prior to the earthquake. Repeating earthquakes can provide afterslip data in offshore areas where resolution from land-based GPS is poor.