A semiconductor device operates in an active mode or a standby mode, and includes a substrate-potential power source line supplying a substrate potential which is higher in a standby mode than in an active mode, and a source-potential power source line supplying a source potential which is lower in a standby mode than in an active mode. During a mode shift from the standby mode to the active mode, a potential equalizing transistor is turned ON to pass a current flowing from the substrate-potential power source line to the source-potential power source line, to reduce the time length needed for shifting from the standby mode to the active mode.