A semiconductor device includes a transistor, a diode, a first detection circuit, a second detection circuit, a calculation circuit, and a determination circuit. The diode is connected in reverse parallel with the transistor. The first detection circuit is configured to detect a change rate of a gate voltage of the transistor with respect to time. The second detection circuit is configured to detect a gate current of the transistor. The calculation circuit is configured to calculate a gate capacitance based on the change rate of the gate voltage with respect to time, and the gate current. The determination circuit is configured to determine, based on a determination result of the gate capacitance when a charge is injected to a gate of the transistor, whether a current flows to the diode or to the transistor.