PURPOSE: To turn off an in-vehicle lamp if a lift gate door is half-open when the speed of an automobile is equal to or greater than a certain speed by connecting a position detecting means to a speed detecting means, and controlling the illumination of the in-vehicle lamp in response to a door half-open signal and a speed signal.

CONSTITUTION: A vehicle speed sensor 14 feeds a voltage signal proportional to vehicle speed to a microprocessor 20 via an input buffer 32. A door position input 12 supplies an open signal from a door half-open switch 40 or 48 to the input buffer 32 when a door is closed, and supplies a ground signal when the door is half-open. The vehicle speed sensor 14 is connected to the door position input 12 to control the illumination of a lamp in the vehicle in response to a door half-open signal and a speed signal. When the speed of the vehicle is equal to or greater than a certain speed, the lamp is turned off if a lift gate door 48 is half-open, to prevent the lamp from being repeatedly turned on and off.