To provide a protection method whereby high load operation can be intermittently performed without interrupting an inverter current.

The protection method for power semiconductor devices (1 to 6) is as follows: a thermoelectric cooling module (27) is installed on a heat sink (7), and the temperature of the thermoelectric cooling module is detected by a thermistor (10) installed thereon. A semiconductor device current is computed from a drive signal for turning on/off the gates of the power semiconductor devices, sent out from a PWM control unit (14), and an inverter output current, and the temperature rise between the thermoelectric cooling module and a junction is estimated from the semiconductor device current. When the junction temperature of the power semiconductor devices, computed from the estimated temperature rise and the thermoelectric cooling module temperature exceeds a preset temperature, the current of an inverter circuit is limited and a voltage command is given to the thermoelectric cooling module to cool the power semiconductor devices.