A power supply system includes a DC-DC converter, a relay, and a control unit configured to i) start up the DC-DC converter based on a state transition of an ignition signal from OFF to ON, ii) cause the DC-DC converter to output a first low voltage when a high-voltage battery is connected to the DC-DC converter by the relay, iii) cause the DC-DC converter to output a voltage specified by an output voltage command in a case where the output voltage command is provided from a high-order ECU after output of the first low voltage is started, and iv) cause the DC-DC converter to output a fixed voltage higher than the first low voltage in a case where the output voltage command is not provided from the high-order ECU by a time at which a first time elapses after the output of the first low voltage is started.