a transformer including a primary coil, a secondary coil, and an auxiliary coil;

a switching element configured to supply or cut off electric power to the primary coil by a switching operation;

a control unit configured to control the switching operation;

a feedback unit configured to feed back a voltage output from the secondary coil; a first rectifying/smoothing portion configured to rectify and smooth an AC voltage input to the first rectifying/smoothing portion, and to output a rectified and smoothed voltage to the primary coil;

a second rectifying/smoothing portion configured to rectify and smooth a voltage induced on the auxiliary coil; and

a voltage divider which includes a voltage divider resistor, and is configured to divide a voltage output from the second rectifying/smoothing portion by the voltage divider resistor, and to output a voltage to the control unit,

wherein the control unit is configured to perform an intermittent control of repeating a switching period for performing the switching operation and a stop period for stopping the switching operation,

wherein the control unit includes a measurement portion configured to measure a length of the stop period,

wherein in the switching period, when the control unit performs the switching operation of the switching element a predetermined number of times, the switching period is transitioned to the stop period, and in the stop period, when the control unit determines, based on a voltage output from the feedback unit, that the voltage output from the secondary coil falls below a target voltage, the stop period is transitioned to the switching period,

wherein the control unit is configured to change the switching operation of the switching element in the switching period based on the length of the stop period measured by the measurement portion, and

wherein the control unit is configured to estimate a voltage value of the AC voltage based on an output voltage of the voltage divider, the length of the stop period, a voltage drop per unit time of the second rectifying/smoothing portion in the stop period, a division ratio of the voltage divider, and a ratio of number of turns between a number of turns of the primary coil and a number of turns of the auxiliary coil.