A light control apparatus for a vehicle is integrated with headlamp driving relays, a light sensor, a lighting switch which selects a manual mode or an automatic mode, and a control unit which controls the relays in accordance with the light sensor output when the lighting switch selects the automatic mode and turns on or off the headlamps. Two power source systems and two headlamp driving systems are provided. The signal level of a given terminal of the control unit is detected and is input to the microcomputer of the control unit. The microcomputer judges whether or not trouble has occurred by detecting a level difference. An alarm indication is given by a lamp or a buzzer if trouble is detected.