In an image forming apparatus including plural exposure units each deflecting a light beam from a light source by means of an oscillation mirror in resonant oscillations and scanning the deflected light beam on a latent image carrier thereby forming a latent image on the latent image carrier, every one of the exposure units assuredly attains an adequate amplitude of the oscillation mirror for ensuring the formation of images of high quality. More specifically, a CPU 101 retrieves resonant frequencies of individual deflectors previously stored in a memory. The CPU 101 calculates an average value of these resonant frequencies and applies a drive command to mirror drivers 1022 of the exposure units 6Y, 6M, 6C, 6K such as to drive all the deflectors into oscillations at the average value. Alternatively, a resonant-frequency adjusting section 653 makes adjustment to match the resonant frequencies of the individual deflectors substantially with a drive frequency. This ensures that all the oscillation mirrors attain adequate amplitudes.