Patent ID: 6876821

Claim:
A color image forming apparatus comprising: a plurality of image forming units having an image carrier and adapted to form a image on the image carrier; a transfer unit adapted to sequentially transfer the images formed by said image forming unit on the image carrier, to a transfer material; a transfer material carrier adapted to serve and transport the transfer material; a first detector and a second detector adapted to detect a pattern image on the transfer material carrier; a pattern forming means for controlling a first image forming unit among said plurality of image forming units to form a reference pattern to be detected by said first detector, and for controlling a second image forming unit among said plurality of image forming units to form a detection pattern to be detected by said second detector; a computing unit adapted to compute an amount of displacement of an image formed by said first image forming unit and an image formed by said second image forming unit based on detection results of said detectors; a phase controller adapted to control a rotational phase of said image carrier based on a computing result of said computing unit.