An image forming apparatus including: a photosensitive member; a process member configured to act on the photosensitive member; and a light scanning apparatus configured to emit a light beam to the photosensitive member. The light scanning apparatus includes: a light source; a rotary polygon mirror; a first signal generating unit configured to receive the light beam deflected by the rotary polygon mirror to generate a first signal indicating a scan starting position; and a second signal generating unit configured to generate a second signal based on a rotation angle of the rotary polygon mirror. A lighting-up timing of the light source for generating the first signal is determined based on a number of the second signals after a voltage for image formation is applied and a relationship between the first signal and the second signal is obtained before the voltage is applied and after the rotary polygon mirror is rotated.
