An image processing apparatus has a semiconductor laser (14) for modulating a light beam (24), a beam detector (5) for detecting the beam, a selector (12) for selecting an output from a digital data input unit (10) or a black signal generator circuit (11), a comparator (14) for pulse-width modulating an output from the selector received through a D/A converter (13), and a signal generator circuit (21) for generating a video enable signal for switching the selector. The black signals are applied through the D/A converter and PWM comparator (14), rather than through an insertion circuit after the comparator, in order to avoid signal degradation caused by such an insertion circuit.