In a multi-color image forming apparatus including a plurality of color image forming units adapted to form a plurality of different color images, a transfer belt to which positioning color image patterns are transferred by the color image forming units, and an optical sensor adapted to detect the positioning color image patterns, a positioning color image pattern trailing edge detecting circuit is provided to detect trailing edges of the positioning color image patterns by determining that an output signal of the optical sensor has reached a threshold value. The threshold value is a predetermined ratio of a peak value of the output signal of the optical sensor. A control circuit is provided to compensate for registration of the color image forming units in accordance with the detected trailing edges of the positioning color image patterns.
High accuracy multi-color image forming apparatus and method for detecting positioning color image patterns