The Hokkaido-Nansei-Oki earthquake (M=7.8) occurred off the south-west coast of Hokkaido, Japan, at 22:17 (JST) on July 12, 1993. We observed its aftershocks by broad-band seismometers at five stations for about four months. Our analysis of the broad-band data indicates that the stress drops of aftershocks have a regional dependency. The stress drops in the central part of the aftershock zone are larger than those in the northern and the southern parts. This result suggests that the stress field in this region is closely related to some previous seismic events that occurred in the vicinity. ¿ American Geophysical Union 1995