Aftershock focal mechanisms of the 1995 Hyogo-ken Nanbu earthquake (Mw=6.9) are analyzed to detect temporal variations in the stress field on the Nojima fault and the Rokko fault system, which ruptured during the mainshock. Following the mainshock, we observe temporal changes in the P axes of aftershocks. Around the Nojima fault, P-axis orientations of aftershocks which occurred in 1996 and 1997 are almost normal to the fault. In contrast, aftershocks which occurred after 1998 have P axes consistent with the regional stress. Around the central part of the Rokko fault system, P-axis directions of aftershocks which occurred in 1996 with focal depths less than 10 km are almost normal to the faults. In contrast, aftershocks which occurred after 1997 have P axes consistent with the regional stress. These results indicate that the Nojima fault and the Rokko fault system started to accumulate significant shear stress in mid-1998, only 3 years after the mainshock. ¿ 2001 American Geophysical Union