An output voltage error detection unit outputs an auxiliary winding voltage generated across an auxiliary winding having the same number of turns as a secondary winding a certain period after a secondary conduction period starts. A correction amount calculation unit calculates a secondary voltage drop caused by a secondary current flowing in the conduction period based on a primary current flowing when the conduction period starts and outputs a calculation result as a correction amount. A reference voltage generation unit generates a reference voltage by adding the correction amount to the output voltage. A control unit generates a feedback signal to minimize the error between the auxiliary winding voltage obtained after a certain delay period and the reference voltage. A PWM signal generation unit controls a PWM signal based on the feedback signal, adjusts switching of the switching element, and maintains the output voltage at a constant level.