PURPOSE: To obtain an image of good quality by adjusting a black dot diameter in a laser beam scanning direction with an optional variation rate, and increasing, decreasing the tone and adjusting half-tones of back dots.

CONSTITUTION: A 1st selecting circuit (A) 17, a 2nd selecting circuit (B) 20, and an LD drive signal generating circuit 18 operates with duty information DUTY from a controller or external control circuit and black data emphasis control information BLKENH and half-tone control information HALFMODE received together with the information DUTY. Consequently, an LD drive signal LDDATA which controls the illumination and extinction time of a laser diode is obtained to control the size of a black picture element of one dot.