When an abnormality is determined in the transition of an ECU (5a-5e) to a power saving state, this power supply control system (1) sequentially turns on current supply switches (19a-19d) while keeping a bypass switch (13) on. A monitoring controller (21) determines an abnormality in the transition of the ECUs (5a-5e) to a power saving state on the basis of whether the dark current in the ECUs (5a-5e) calculated from the amount of change at that time in the current (Isens) in the power supply line (7) is greater than the normal dark current. Further, in the startup state of the ECUs (5a-5e), power from a power source (VB) can be supplied to the ECUs (5a-5e) via the power supply line (7) and individual supply lines (9a-9d).