Corrective signals for canceling out a dynamic deformation of the reflecting surface of a rotary scanning mirror are applied to X and Z electrodes of an electro-optic lens. Based on the supplied corrective signals, the electro-optic lens controls the refractive index of a light-transmissive element to refract a laser beam in a direction to eliminate an excessively converged or diverged condition of the laser beam due to the dynamic deformation of the reflecting surface of the rotary mirror. The spatial vector of the corrective signals supplied to the electro-optic lens rotates in synchronism with the rotation of the rotary scanning mirror. Therefore, the laser beam which passes through the electro-optic lens and is reflected by the reflecting surface forms a beam spot on a film, which stays focused at all times irrespective of the angular displacement of the rotary scanning mirror.