PURPOSE: To accelerate the adjustment in the quantity of light by providing photodetecting means for detecting backwardly emitting beam at the rear of an array light source formed of a plurality of semiconductor lasers to detect the peak values of the respective lasers and adjusting the quantity of light so that the peak values become equal.

CONSTITUTION: A CCD detector 3 for detecting the backward emitting beam is arranged at the rear of an array light source 1 formed of a plurality of semiconductor lasers. To detect and equalize the peak values of the respective lasers, the output signal from a CCD detector 11 is inputted to line memory 13 through an A/D converter 12 as shown, and the output is detected by a peak detector 14 and is compared by a comparator 15 with the peak value to be referenced. Thereafter, the difference is obtained by a diference detector 16, a current value corresponding to the quantity of light to be corrected is obtained by a converter 17, and a laser driver formed of digital control power sources 19a∼19c is operated by a multiplexer 18.