An electric power supply device includes a first battery (11) electrically connected to each of a plurality of load instruments mounted on a vehicle and supplying electric power to each of the load instruments, a second battery (12) electrically connected to the first battery (11) and each of the load instruments via a switch (SW1) and capable of supplying electric power to each of the load instruments, and an ECU (13) configured to cut off electrical connection between the second battery (12) and each of the load instruments by turning OFF the switch (SW1) on the condition that a high-load instrument (21) as one of the load instruments is not in operation.