PURPOSE: To prevent overheating within a housing with adjustment of the temperature therein by keeping constant the intensity of illumination in main instruments necessary for safe running while that of auxiliary instruments is controlled to reduce or extinguish according to temperature.

CONSTITUTION: A battery powr source Vcc is supplied to a main lamp 5 with a lighting switch SW "ON" to light a speedometer 1 or the like as main instrument at a fixed intensity while supplied to a sub-lamp 6. The output voltage Vout of a lighting control circuit 7 is determined by a partial voltage between the parallel resistane of a resistance R2 and a thermistor resistance Rt and a resistance R3 and when the resistances R2 and R3 are constant, the voltage Vout varies with the resistance Rt. In other words, as the temperature within an instrument housing rises, the thermistor resistance Rt reduces to lessen the output voltage Vout causing the sub lamp 6 to be dimmed. This inhibits the rise in the temperature within the housing due to a lamp heat thereby preventing the deformation of synthetic resin and malfunctioning of an electronic circuit due to overheating.