Abstract: A set pattern image with no displacement is recorded onto recording paper. In this set pattern image, the centers of auxiliary patterns are displaced in an auxiliary scan direction off a referential straight line passing through the centers of primary patterns. The image forming apparatus causes the set pattern image to be read, measures the displacements of the auxiliary patterns in the auxiliary scan direction off the referential straight line, and compensates for the displacements, such as by adjusting writing timings in the auxiliary scan direction so that the displacements become zero.

Abstract: Assuming the photosensitive drum for black as a reference, the phase of the adjacent photosensitive drum for cyan leads by about 60 deg. Similarly, the phase of the photosensitive drum for magenta leads by about 120 deg., and the phase of the photosensitive drum for yellow leads by about 180 deg. The phase of each photosensitive drum is shifted to thereby make it possible to shift the phase of driving unevenness. By shifting the phase, even the distance between adjoining transfer positions corresponding to the image forming stations is set shorter than the circumference of the photosensitive drum, the variation of each photosensitive drum due to driving unevenness with respect to the printing medium passing through the transfer position can be kept congruent with the others.

Abstract: In a color image forming apparatus, component color images formed by respective image forming stations are sometimes displaced as they are transferred onto recording paper. Therefore, a set pattern image (Q0) with no displacement at all is recorded onto recording paper as a set pattern image (Q1). In this set pattern image, the centers of auxiliary patterns (Y1, C1, M1) are displaced in an auxiliary scan direction off a referential straight line (H) passing through the centers of primary patterns (K1, K1′). The image forming apparatus causes the set pattern image to be read, measures the displacements (&Dgr;Y1, &Dgr;C1, &Dgr;M1) of the auxiliary patterns in the auxiliary scan direction off the referential straight line, and adjusts writing timings in the auxiliary scan direction so that the displacements become 0. As a result, the color image recorded on the recording paper shows improved quality.