A power conversion device has a main switch connected to a positive electrode side line of a battery, at least one semiconductor power element connected between the main switch and a negative electrode voltage line, and an auxiliary circuit having an auxiliary switch, a resonance reactor and an auxiliary diode connected parallel to the main switch. The device further has a voltage detection part which detects an external side voltage of the main switch and a control circuit part performing a switching control of the main switch and the auxiliary switch based on the detection result of the voltage detection part. During the turned-off state of the main switch, the control circuit part turns off the auxiliary switch before the external side voltage of the main switch reaches an input voltage of the main switch. This external side voltage starts to increase when the auxiliary switch is turned on.