According to one embodiment, a semiconductor device includes: a first switch SWx which switches whether or not to supply a first power supply voltage Vx generated by accumulating a charge outputted from a power source 10, as a second power supply voltage VDD to a first circuit 13, and a second switch SW1 which switches whether or not to connect to the first circuit 13 a smoothing capacitor C1 which suppresses a fluctuation of the second power supply voltage VDD, and the first switch SWx is switched to an on state in response to that the first power supply voltage Vx has reached a sufficient voltage, and then the second switch SW1 is switched to the on state in response to that the second power supply voltage VDD has reached a sufficient voltage.