PURPOSE: To prevent influence of jitter of a motor automatically and to facilitate adjustment of a scanning device by counting scanning start pulse period sections, reading the counted value by a jitter correcting circuit, calculating the deviation and correcting sampling frequency for each face.

CONSTITUTION: Laser light from the scanning light source 3 of a scanning device is irradiated onto the face 17 of an original through a rotary multilateral mirror 1, a lens 15 and a photoelectrical conversion mirror 16. Scanning start pulse from a scanning start pulse generator 7 and scanning area information of a scanning face discriminating circuit 9 are inputted to a phase section counter 19, phase section of each face of the multilateral mirror 1 is counted by output of a reference pulse generator 18, and the counted value is added to a CPU10. This is compared with a reference counted value read out from an RAM12 by the CPU10, and correction value for sampling frequency is calculated. The correction value is added to a sampling pulse generator 6, and correction for each face of the multilateral mirror 1 is made. Thus, influence of jitter of a main scanning motor is prevented automatically.