A power supply system includes: a first power storage device; a second power storage device having a lower voltage than the first power storage device; a DC-DC converter including a choke coil, a first switching element, a diode connected in parallel with the first switching element, and a second switching element; a semiconductor relay configured to switch a connection state between a second end of the choke coil and the second power storage device; and a controller configured to perform PWM control of the first switching element and the second switching element to control ON and OFF of the semiconductor relay. When an ON time of the second switching element is controlled to become zero and a current flowing out from the second power storage device exceeds a first reference current, the controller reduces a duty ratio of an ON time of the first switching element.