To accurately and simply adjust a read scanning start position and a read scanning time to correct a 'tip resist deviation' by storing calculated error correction data and adjusting the read scanning start position and the read scanning time of a read scanning section based on the stored error correction data.

A 'tip resist deviation' X that is an error in the read start position is produced in a subscanning direction A due to a device error between a read image based on read image data D1 and a reference image based on reference image data D2. In the case that an error in the read start position of the reference original is detected in the read image data D1 and the reference image data D2, error correction data to correct the error of the read start position are calculated based on the error. A read scanning start position and a read scanning time of a read scanning section are adjusted based on the error correction data. Thus, the read scanning start position and the read scanning time of the read scanning section are adjusted by only having to read and scan the reference original only once.