PURPOSE: To accomplish proper correcting of a picture of a fine line etc. in the main scanning direction by controlling the effective-beam diameter of a laser beam based on the content of a picture element signal neighboring the subscanning direction.

CONSTITUTION: A control part 21 rotate a rotary polygon mirror 1 by controlling a motor drive part 23 that drives a motor 2 at the prescribed number of revolution, as well as actuates a laser drive part 24 in synchronization with the input-start-up timing of a picture signal, to start up a laser generator 25 to operate. The part 21 further controls to output the input picture signal in synchronization with the rotation of the mirror 1 to an AOM drive part 27 that drives an acoustic optical modulator (AOM) 26. Accodingly, a laser beam modulated by using the picture signal is outputted from the AOP 26, and is irradiated on the mirror 1. At this point, the effective-beam diameter is detremined depending on the content of the picture element neighboring the subscanning direction, which becomes smaller when recording a fine line in the main scanning direction, and hence its picture is made to be better displayed.