In general, according to one embodiment, a semiconductor device includes a device main body, a semiconductor substrate. The device main body includes a semiconductor substrate mounting part and a first conductor provided around the semiconductor substrate mounting part. The semiconductor substrate includes a DC-to-DC converter control circuit having a detector to detect at least one of a current flowing through the first conductor and a voltage supplied to the first conductor. The semiconductor substrate is disposed on the semiconductor substrate mounting part so that the detector comes close to the first conductor.