A power supply system comprising: 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 (L), a first switching element (S1) configured to switch a connection state between a first end of the choke coil and the first power storage device, a first diode (D1) that is connected in parallel with the first switching element (S1) and that rectifies a current flowing from the choke coil side to the first power storage device side, and a second switching element (S2) configured to switch a connection between a ground and the first end of the choke coil; and a controller configured to perform pulse width modulation control of the first switching element (S1) and the second switching element (S2), a semiconductor relay configured to switch a connection state between a second end of the choke coil and the second power storage device , wherein the controller is configured to control ON and OFF of the semiconductor relay , and wherein the controller is configured to reduce an ON time of the first switching element (S1) when an ON time of the second switching element (S2) is controlled to become zero, a current flowing out from the second power storage device exceeds a first reference current, in a state in which a voltage on the first power storage device side becomes lower than a voltage of the second power storage device, and the voltage difference between the voltage on the first power storage device side and the voltage of the second power storage device is within a range of a drop voltage due to the first diode (D1). 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 (L), a first switching element (S1) configured to switch a connection state between a first end of the choke coil and the first power storage device, a first diode (D1) that is connected in parallel with the first switching element (S1) and that rectifies a current flowing from the choke coil side to the first power storage device side, and a second switching element (S2) configured to switch a connection between a ground and the first end of the choke coil; and a controller configured to perform pulse width modulation control of the first switching element (S1) and the second switching element (S2), a semiconductor relay configured to switch a connection state between a second end of the choke coil and the second power storage device , wherein the controller is configured to control ON and OFF of the semiconductor relay , and wherein the controller is configured to reduce an ON time of the first switching element (S1) when an ON time of the second switching element (S2) is controlled to become zero, a current flowing out from the second power storage device exceeds a first reference current, in a state in which a voltage on the first power storage device side becomes lower than a voltage of the second power storage device, and the voltage difference between the voltage on the first power storage device side and the voltage of the second power storage device is within a range of a drop voltage due to the first diode (D1). <EOS>