An image processing apparatus includes a first sensor having a plurality of reading elements arranged in the primary scanning direction, a second sensor disposed in parallel with the first sensor and by a predetermined lines in the secondary scanning direction, an integral correction portion for correcting a time difference of data output due to a position difference between the first sensor and the second sensor by a line unit, and a fractional correction portion for correcting a time difference of data output due to a position difference between the first sensor and the second sensor by a sub line unit. Thus, the correction of the phase shift between element arrays can be performed as precisely as possible and the reproducibility of a black fine line is increased.