A waveform inversion was conducted for the rupture process of the 2004 Mid Niigata Prefecture (Niigata-ken Chuetsu) earthquake (M_J 6.8) by using aftershock records as Green's functions. Paying attention to the similarity of the group delay time between the mainshock and aftershock records, two aftershocks were selected to be used in the inversion. As a result of the inversion, a rupture model was obtained that can explain mainshock records at periods from 1 to 5s. Although only portions of 10s including S wave were used for the inversion, the later phases, which were not used for the inversion, can also be explained fairly well with the present rupture model. Another inversion, in which only one aftershock was used, was also carried out. In this case, the mainshock recordings were not explained very well, indicating the neccesity of at least two aftershocks in the analysis. The primary asperity found in the present rupture model is close to the heavily-damaged area in Tamugiyama, which was found by the investigation team of the Niigata University.