PURPOSE: To eliminate jitter due to an angle error of a rotary polygon mirror, by starting storing a readout signal in response to the scan starting pulse of each surface of the rotary polygon mirror, and then start reading the stored signal in response to the scan starting pulse of a following surface.

CONSTITUTION: Light emitted from a scanning light source 2 is made incident to a rotary polygon mirror 1 to make a scan on an original 12. Reflected light from the original 12 is made incident to a photoelectric converting element 31, whose output is inputted as a picture signal to a delay circuit 6. A photosensor 41 is provided at the end part where the scan on the original 12 is started, and generates an output pulse when the reflected light from the rotary polygon mirror 1 is made incident. The output pulse is generated when the scan on the polygon mirror is started. The picture signal obtained by the scanning is written in the line memory of the delay circuit 6 and read when a scan starting pulse corresponding to a following surface is inputted. Then, an angle error of the polygon mirror is eliminated to improve the picture quality.