PURPOSE: To obtain an image signal without image distortion even if respective sampling points are a little deviated in a subscanning direction caused by using a light deflection unit whose precision is inferior by obtaining a value corresponding to the image signal in the case where unevenness in scanning does not exist with interpolation from the image signal read by using scanning unevenness information in the subscanning direction.

CONSTITUTION: Laser beam 2 reflected by a half mirror 5 is used as monitor light 2c having the information of the deviation in position in the subscanning direction and the variation of the incident position, where the monitoring light 2c is made incident on a light receiving device 18, in a B direction corresponds to that of the stimulating light 2a, caused by the distortion, etc., of a polygon mirror 5, in the subscanning direction (C direction). Then it is detected that the information how much it is deviated in the subscanning direction from an original sampling position is sampled so as to calculate the value of the image signal on the original sampling position with the interpolation from the value of the image signal on said deviated position by an arithmetic processing means 22. Thus, the image signal without the image distortion can be obtained even if the respective sampling points are a little deviated in the subscanning direction caused by using the light deflection unit whose precision is inferior.