To shut off dark current flowing from a battery to an ECU (engine control unit) while electrically connecting the battery to an ECU.

The power supply device for an internal combustion engine includes: an alternator 1; the battery 3 connected to the alternator 1; a starter motor 7 driven by the battery 3; the ECU 4 connected by branching from the connection line of the alternator 1 and the battery 2; and an engine load 5 connected to the ECU 4. A battery relay 12 controlled to be opened or closed by the ECU 4 is arranged, as a shut down means for shutting down the power supply to the ECU 4 from the battery 3 in the operation stop of the internal combustion engine, in the connection line of the alternator 1 and the battery 3 and on the side of the battery 3 rather than the branch point to the ECU 4. The battery relay 12 is constantly opened, and is controlled to be closed by the ECU 4 when the voltage of the battery exceeds the threshold value.