Patent ID: 8004546

Claim:
A deviation amount detecting device which computes an amount of deviation for each of multiple toner images of different colors in an electrophotographic color image forming device wherein a color image is formed on a transporting member by superimposing the toner images of different colors, the deviation amount detecting device comprising: an image formation unit configured to form on the transporting member a set of second deviation detecting patterns which are of different colors and of identical shape and superimposed at a same position in order to detect a deviation for each of toner images of the different colors; a pattern reading unit configured to read the set of second deviation detecting patterns formed on the transporting member by the image formation unit; and a detection unit configured to detect whether a deviation for each of the toner images of the different colors on the transporting member takes place, based on position information which is stored as a result of the reading of the set of second deviation detecting patterns by the pattern reading unit, wherein the image formation unit is further configured to form, on the transporting member, a set of first deviation detecting patterns which are of different colors and have different shapes, and an area of the transporting member on which the set of second deviation detecting patterns are formed is smaller than an area of the transporting member on which the set of first deviation detecting patterns are formed, wherein the set of first deviation detecting patterns are formed on the transporting member by the image formation unit when the detection unit detects that the deviation takes place, wherein the set of second deviation detecting patterns are formed on the transporting member by laser beams which pass through lenses located at opposite positions around a center of a polygon mirror in an exposure unit of the image forming device and disposed in a vicinity of a drive motor which drives the polygon mirror, and wherein the lenses are two deflector lenses which are disposed in a vicinity of the polygon mirror in the exposure unit.