A power supply device (100, 200) including a power factor correction circuit and configured to supply power to a load, the power supply device comprising: an AC power supply configured to output an AC voltage between a first input terminal and a second input terminal; a rectifier circuit connected between the first input terminal and the second input terminal and configured to output, between a first power supply terminal and a second power supply terminal, a power supply voltage obtained by rectifying the AC voltage supplied from the AC power supply; a first load terminal connected to a high potential side terminal of the load and a second load terminal connected to the second power supply terminal and a low potential side terminal of the load; an input side voltage detection circuit connected between the first power supply terminal and the second power supply terminal, and configured to detect an input side detected voltage based on the power supply voltage; an output side voltage detection circuit connected between the first load terminal and the second load terminal, and configured to detect an output side detected voltage based on an output voltage between the first load terminal and the second load terminal; a primary side winding having one end that is connected to the first power supply terminal, the primary side winding being included in a transformer; a secondary side winding having one end that is connected to the second power supply terminal, the secondary side winding being included in the transformer; a main switch having one end that is connected to another end of the primary side winding and another end that is connected to the second load terminal; characterized in that an error amplifier configured to output an error voltage based on a difference between the output side detected voltage and a predefined reference voltage; a subtractor configured to output a subtraction result signal based on a result of a subtraction of the input side detected voltage from the error voltage; an output comparator configured to output a comparison result signal based on a result of a comparison between the subtraction result signal and a timer signal; and a flip-flop circuit having a set terminal to which a signal from another end of the secondary side winding is inputted, a reset terminal to which the comparison result signal is inputted, and an output terminal from which a control signal for turning on or off the main switch is outputted. an AC power supply configured to output an AC voltage between a first input terminal and a second input terminal; a rectifier circuit connected between the first input terminal and the second input terminal and configured to output, between a first power supply terminal and a second power supply terminal, a power supply voltage obtained by rectifying the AC voltage supplied from the AC power supply; a first load terminal connected to a high potential side terminal of the load and a second load terminal connected to the second power supply terminal and a low potential side terminal of the load; an input side voltage detection circuit connected between the first power supply terminal and the second power supply terminal, and configured to detect an input side detected voltage based on the power supply voltage; an output side voltage detection circuit connected between the first load terminal and the second load terminal, and configured to detect an output side detected voltage based on an output voltage between the first load terminal and the second load terminal; a primary side winding having one end that is connected to the first power supply terminal, the primary side winding being included in a transformer; a secondary side winding having one end that is connected to the second power supply terminal, the secondary side winding being included in the transformer; a main switch having one end that is connected to another end of the primary side winding and another end that is connected to the second load terminal; characterized in that an error amplifier configured to output an error voltage based on a difference between the output side detected voltage and a predefined reference voltage; a subtractor configured to output a subtraction result signal based on a result of a subtraction of the input side detected voltage from the error voltage; an output comparator configured to output a comparison result signal based on a result of a comparison between the subtraction result signal and a timer signal; and a flip-flop circuit having a set terminal to which a signal from another end of the secondary side winding is inputted, a reset terminal to which the comparison result signal is inputted, and an output terminal from which a control signal for turning on or off the main switch is outputted.