Patent ID: 8548347

Claim:
A color image forming apparatus that forms test patterns of a plurality of colors on an image carrier, detects a shift of each of the test patterns of the plurality of colors formed on the image carrier, and corrects a position of an image of each of the plurality of colors to be formed on the image carrier based on the shifts of the test patterns of the plurality of colors, the apparatus comprising: a test pattern formation portion that forms the test patterns of the plurality of colors in an image formation area in which images of the plurality of colors are formed on the image carrier and in an image non-formation area provided on both sides of the image formation area on the image carrier; a detection portion that detects a shift of each of the test patterns of the plurality of colors formed in the image formation area and a shift of each of the test patterns of the plurality of colors formed in the image non-formation area; a computing portion that divides the test patterns in both edge portions of the image formation area and the test patterns in the image non-formation areas into two groups each made up of adjacent test patterns and computes a shift between the test pattern in the image non-formation area and the test pattern in the image formation area; and a storage portion that stores the shift computed by the computing portion.