According to one embodiment, there is provided a semiconductor device comprising: a control circuit connected to a bus; a first circuit operating under control of the control circuit; a bus access detection circuit that detects bus access from the control circuit to the first circuit without going through the bus; a switch element connected between the first circuit and a power supply; and a second circuit connected between the first circuit and the bus, the second circuit controlling, when the bus access to the first circuit is detected by the bus access detection circuit, the switch element such that power from the power supply is supplied to the first circuit.