A DC-DC converter includes a series circuit including a primary transformer coil and a main switch element connected between a power input terminal and a ground terminal. A secondary transformer coil is connected to a rectifying/smoothing circuit including rectification-side and commutation-side synchronous rectifiers, a smoothing capacitor, and a choke coil. The output voltage from the rectifying/smoothing circuit is supplied to a load connected to a power output terminal. An input voltage detection circuit detects the voltage between the power input terminal and the ground terminal, and supplies a detection signal to a VIN terminal of a switching control circuit. The switching control circuit performs PWM control to maintain a constant output voltage output to the load, reduces the switching frequency when the input voltage input to the VIN terminal is low, and increases the switching frequency in accordance with an increase in the input voltage.