POWER SUPPLY CONTROL DEVICE

A power supply control device is applied to a vehicle equipped with electrical equipment, a relay connected to the electrical equipment, a first power supply that makes the relay operable, and a second power supply that discharges electricity to the electrical equipment via the relay. The device includes: a DC/DC converter that supplies electric power from the second power supply to the relay, serving as a backup for the first power supply; a third power supply charged by the first power supply, outputting the charged electric power to the relay; and a control unit that controls the DC/DC converter and second power supply. The control unit turns off the DC/DC converter when the third power supply's voltage is equal to or larger than a first threshold value and turns on the DC/DC converter when the third power supply's voltage is equal to or smaller than a second, lower threshold value.