A power supply unit includes a switching circuit that drives a switching device by a duty ratio corresponding to a target voltage output value and outputs an alternating current signal; a coupling capacitance element provided in a subsequent stage with respect to the switching circuit; a voltage transformation unit that increases a voltage of the alternating current signal input by using the switching circuit, and outputs a high-voltage alternating current voltage to an electrifying unit; a charging unit that charges the coupling capacitance element in response to a power supply voltage being turned on; and a control unit that controls driving of the switching device to cause the coupling capacitance element to be charged to a predetermined charged amount before the switching circuit starts outputting the alternating current signal.