We investigated the rupture process of the 1993 Hokkaido-Nansei-Oki earthquake (M_JMA=7.8) using the broad-band strong motion seismograms recorded at Tomari which is located about 260km distant from the epicenter. We determined the rupture model employing the empirical Green function method in which the waveform of a main shock is synthesized from the waveform of an aftershock. We used the aftershock (M_JMA=6.0) that occurred near the main shock epicenter about three hours later. The observed waveforms are best fitted to the synthetics produced for the rupture model involving two major events which ruptured at a time interval of 31-34 seconds. The first event is located near the epicenter and the second event is located near the Okushiri island, about 70km south of the first event. The fault sizes of the two events are both 40×20km², which are small as compared with the overall area of the aftershock distribution. The total rupture duration is 43-46 seconds. It appears likely that the seismic energy was not released uniformely from the entire aftershock area. The present rupture model is consistent with the previous results obtained from the waveform inversions of teleseismic and strong motion data, suggesting the validity of the empirical Green function method and its usefulness for investigating the rupture process of large earthquakes.