The present invention aims to avoid problems such as not detecting line-shaped noises or erasing images on the sheet by erroneously detecting line-shaped noises. A copy machine 1 refers to a judgment table 50b to determine a color corresponding to values of RGB color components read in Step S101 (Step S102). The copy machine 1 selects thresholds and coring levels corresponding to matched RGB color components (Step S104). The copy machine 1 sets the selected thresholds and coring levels to an image processing unit 48 (Step S106). The copy machine 1 judges whether a difference between RGB color components of a target pixel and its neighboring pixels is greater than the thresholds that has been set in Step S106 (Step S107). If judging affirmatively, the copy machine 1 stores the address of the target pixel as a line-shaped noise candidate pixel into a line-shaped noise address storing area 49b (Step s108), and extract a series of line-shaped noise candidate pixels as line-shaped noise pixels (Step S110).