We performed waveform inversion of long period body wave data to investigate whether it is possible to determine the moment release distribution in time and space, using the 2003 Tokachi-Oki earthquake as an example. Our results show that it is difficult to determine the precise spatial distribution, and therefore we focused on determination of the temporal distribution. We allowed changes of the source mechanism during rupture in the inversion scheme. The temporal change of the scalar moment of the subevents was consistent with the moment rate function inferred from other studies. The source mechanisms of the subevents were primarily reverse faults, matching preliminary reports of this earthquake. No significant change of the source mechanism during the rupture were inferred. Our results suggest that the rupture process of this earthquake was rather simple.