VEHICLE POWER SUPPLY SYSTEM

The vehicle power supply system (1A) includes the DC/DC converter (16) that is disposed on an electric path between the first load (14) related to vehicle control and the battery (11) and converts a voltage of direct current power supplied from the battery (11) to the first load (14), and the detection device (21) that detects the voltage of the battery (11). In a case where the engine (2) is restarted by using the starter (13) from the automatically stopped state of the engine (2), the detection device (21) controls the DC/DC converter (16) to start in a case where the voltage of the battery (11) becomes equal to or less than a threshold value during a period from stoppage of the alternator (12) to start of the starter (13).