A semiconductor laser is caused to emit laser light continuously on a plurality of scanning lines prior to image formation in a laser printer. An electric current to be applied is controlled so that the detected light quantity from the semiconductor laser becomes equal to the desired light quantity. The light quantity detected during pulsed emission with the applied current is used as the desired light quantity for APC which is performed at a space between sheets when a non-image area is scanned. Alternatively, laser light is emitted on a plurality of lines before the start of printing when printing of multiple pages is to be performed in a laser beam printer, to perform light quantity control. Then, laser light is emitted during scanning of a nonimage area between pages, to perform light quantity control. When light quantity control is performed between sheets, a value obtained by correcting an error between the quantities of light in both types of light quantity control is set as the desired light quantity. Alternatively, laser light is emitted continuously on a plurality of scanning lines prior to image formation to perform light quantity control. Then, laser light is emitted only when laser is scanning a non-image area to perform light quantity control, thereby setting a current volume to be applied to the semiconductor laser during image formation.