There are provided a power supply unit control device for an internal combustion engine driven vehicle provided with a lamp driving switch for turning lamp-driving current supplied to a vehicle headlamp from a battery charged by the output of an AC generator driven by an internal combustion engine and with a conducting control unit for controlling the lamp driving switch. The conducting control unit normally performs on/off control of the lamp driving switch at a normal duty ratio, the normal duty ratio being an on/off duty ratio for the lamp driving switch at which lamp-driving current can be kept at a value not impeding the charging of the battery, and, when the internal combustion engine decelerates, performs engine deceleration lamp conducting control, in which on/off control of the lamp driving switch at the normal duty ratio is interrupted and the lamp driving switch is controlled so that lamp-driving current necessary to suppress reductions in the brightness of the headlamp is sent to the headlamp, only for a limited length of time.