To correct distortion in an optical system of an optical scanning, electrophotographic image generating apparatus, a parallel line group 107′ is printed by a printing apparatus to be calibrated on a sheet on which a correction pattern 101 has been printed. The user reads marker positions to find amounts of deviation of the adjustment pattern from the correction pattern and inputs the amounts of deviation in the printing apparatus. The printing apparatus interpolates values between the input amounts of deviation, establishes positions at which a line in the adjustment pattern deviate one pixel in a sub-scanning direction as scan line changing points and generate new conversion information by pairing the scan line changing points with respective directions of deviation. During image generation, the printing apparatus prints an image by correcting image data according to the conversion information.