A current fluctuating due to a load fluctuation is limited to protect a semiconductor switch. A protection circuit includes a switch circuit that turns on when a predetermined conduction voltage is applied thereto, and a sub-reactance circuit having a predetermined reactance value is connected in parallel to a main reactance circuit through which a high frequency current generated by a semiconductor switch flows. When the switch circuit is turned on, the main reactance circuit and the sub-reactance circuit are connected in parallel, and a high frequency current flows through this parallel connection circuit. The impedance value of the parallel connection circuit is set to be larger than the impedance value of the main reactance circuit so that the current is limited due to the turning on of the switch circuit, and thus, the semiconductor switch is protected.