a full - color image forming apparatus forms a full - color image using a color - matching pattern of color components formed on an intermediate transfer belt . the image forming apparatus includes at least one pattern detection sensor that detects the color - matching pattern and generates pattern detection signals . a seam mark - detecting sensor detects a seam mark formed on the intermediate transfer belt and generates seam mark detection signals . the seam mark - detecting sensor is aligned with the pattern detection sensor on a same axis extending vertically relative to a traveling direction of the intermediate transfer belt . a memory stores digital signals converted from the pattern detection and seam mark detection signals . a seam mark detection signal deleting device deletes a prescribed seam mark detection signal from the pattern detection signals when the pattern detection signal and the prescribed seam mark detection signal occur simultaneously . a controlling device controls the image forming apparatus in accordance with the pattern detection and seam mark detection signals excluding the prescribed seam mark detection signal .