A protection circuit of a semiconductor device includes a high electron mobility transistor and a protection element. Between the drain and the gate of the high electron mobility transistor, the protection element includes: a thyristor; and a first resistor connected in series to the thyristor. Between the source and the gate of the high electron mobility transistor, the protection element includes: a second resistor and an interrupter that is connected in series to the second resistor. The interrupter interrupts a flow of a current between the drain and the gate when the thyristor is turned off, and the interrupter permits the current to flow between the drain and the gate when the thyristor is turned on.