To provide an exposure means capable of extending the life of equipment by shortening the lighting time of a laser diode and capable of securing sufficiently enough time for the sampling operation of automatic power control even when one scanning time is shortened.

The exposure means comprises a laser driving means, a brushless DC motor, a rotary polygon mirror for reflecting a plurality of modulated laser beams from a plurality of laser light sources by a reflection surface to scan a surface to be scanned, a photoreceptor for exposing a latent image and forming an image by receiving a plurality of modulated scanning beams from the rotary polygon mirror, and an optical sensor for detecting the light outputs of these laser beams and converting the light outputs into electric signals. In a period that the surface of the photoreceptor is not scanned by the lasers, the light quantity of one of these lasers is detected by the optical sensor, sample holding control for determining the constant current value of the laser concerned is performed when the laser is driven by the laser driving means so that the light quantity of the laser reaches target light quantity. In one scanning period, the sample holding control of one of a plurality of laser light sources is performed.