To enable a control unit for controlling a variable valve train, disposed independently of an engine control unit, to execute various types of processing while maintaining power supply after an ignition key is turned off.

Electric power is supplied to both of a microcomputer 402 in the engine control unit 400 and a microcomputer 202 in the variable valve control unit 200, via a unit power relay 304 driven by a relay drive circuit 403 provided in the engine control unit 400. Upon completion of processing after the ignition key is turned off, the variable valve control unit 200 then transmits a power supply shutoff enable signal to the engine control unit 400. The engine control unit 400 turns off the unit power relay 304, provided that processing to be executed after the ignition key is turned off has been completed with the power supply shutoff enable signal already received.

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