PURPOSE: To eliminate the need for the adjustment of a modulation wave level of a modulation circuit applying width modulation of a laser beam and to make the operation stable.

CONSTITUTION: An analog video signal outputted from a D/A converter 2 is compared with a triangle wave generated from a triangle wave signal generating circuit 3 at a comparator circuit 4, and a width modulation wave for modulating a laser beam is generated by a modulation circuit. Then the modulation circuit is provided with a peak hold circuit 10 holding a peak value of the triangle wave and a reset circuit 9 revising the stored peak value while resetting at a prescribed period, and the latched peak level is used for a reference voltage or a D/A converter 2 to make the level ratio of the analog video signal subjected to D/A conversion constant with respect to a fluctuating triangle wave level.