PURPOSE: To provide a light quantity controller which variably controls the light quantity of a laser beam a laser diode emits and in addition, facilitates the setting work of the reference light quantity of the laser beam.

CONSTITUTION: The high voltage of high voltage power supply 24 becomes the reference voltage of a desired voltage value by lowering it by variable resistance 20 and resistance 21, and in addition, by adjusting the resistance value of the variable resistance 20 being a signal intensity setting means. The reference voltage and monitor voltage proportional to the light output of the laser diode 1 are impressed to a comparator 17, and a difference signal is outputted. A light quantity control part 30 feedback and controls the light output based on the difference signal.