PURPOSE: To control the quantity of light of a copying machine and the like accurately, by separating a space having a light receiving element, which is provided in an aenclosing part, from another lighted space, thereby preventing the contamination of the light receiving surface of the light receiving element due to particle dust.

CONSTITUTION: The contact surface to a lamp 2 is closely contacted with the outer surface of a light measuring part 3 of the lamp 2 by using a heat resisting sealing material 9. A light receiving element is attached in the part. A space having the light receiing element is separated from another space at the light measuring part for the lamp 2 and a mirror 1. With respect to the operating state, the change in the quantity of light emitted from the lamp 2 is detected by the light receiving element and trasduced into an electric signal. The signal is transmitted to a control device and processed. Then a power source circuit for the lamp is controlled and the quantity of the light emitted from the lamp 2 is adjusted. The heat in an enclosing part, which is yielded by separating the light receiving part from the other space by the light receiving element enclosing part 7, is always radiated by a fin 8. The overheat of the light receiving element in the light receiving element enclosing part 7 can be prevented.