By correcting for seasonal noise contained in GPS data, we are able to detect small but compelling post-seismic deformation following a medium-sized interplate earthquake (2001 August 14, MW 6.4) that occurred off the Pacific coast of northern Honshu, Japan. Its decaying-time constants are similar among the stations with an average of 2.2 months. We inverted for the slip distribution, assuming that post-seismic deformation was due to a slow slip near the main shock fault. Slow slip spreads mainly over the northern periphery of the northern asperity of the 1968 Tokachi-Oki earthquake (MW 8.2) that has been considered a seismic gap since 1994. The seismic moment of the slow slip is about three times greater (MW 6.7) than that of the co-seismic slip. The present case suggests that the post-seismic slow-slip is common not only for the large earthquakes but also for relatively small interplate earthquakes in this region.