A laser scanner is adapted to cause a laser beam to scan in the main-scanning direction by deflecting the laser beam by means of a deflecting optical system. A horizontal synchronizing signal is obtained by detecting the laser beam by means of a photodetector. Forced emission of laser beam is started after a predetermined time period since the detection of the laser beam, which predetermined time period is variable as a function of the rotary speed of the deflector section. With this arrangement, a horizontal synchronizing signal can be reliably detected if the rotary speed of the deflecting operation system fluctuates.