PURPOSE: To detect accurately luminous quantity of light and to relieve the load of the operator, by controlling a applied power to a lamp so as to keep the quantity of light of the lamp to a prescribed value, in a dimmer of lamp for a copying machine or an optical character reader.

CONSTITUTION: The luminous quantity of light of the lamp 1 is converted into a voltage at a photodetector 4, and this voltage is compared with a predetermined reference voltage at a comparator circuit 5. The compared output is given to a switch circuit 7 using a thyristor via a phase signal generating circuit 6 so as to change the phase of the power supply voltage transmitted from a power supply 8 and to control a power applied to the lamp 1.