A scan-beam detection signal Hsync provided by horizontal synchronous sensors 60A, 60B is inputted to a drive signal controlling section 10Y as a drive period signal related to a drive period of a deflective mirror. The drive signal controlling section 10Y controls a drive signal Sd such as to establish a predetermined relationship between a phase of a reference signal Sr and a phase of the horizontal synchronous signal Hsync, whereby a deflector 65 is allowed to operate in synchronism with the reference signal Sr. Therefore, a latent-image forming position with respect to a sub-scan direction Y is adjusted by way of the phase adjustment between the reference signal Sr and the horizontal synchronous signal Hsync, so that a linear latent image is prevented from deviating with respect to the sub-scan direction Y.