An image forming apparatus (1, 201) comprising: an image forming unit (11, 12, 13, 14) that forms a developer image to be transferred to the belt (23, 123, 223); a transfer belt unit (10, 210) comprising a belt (23, 123, 223), a driving roller (20, 222) provided on an inner side of the belt (23, 123, 223) and driving the belt (23, 123, 223), a supporting roller (21, 22, 220, 221) provided on the inner side of the belt (23, 123, 223) and applying tension to the belt (23, 123, 223); a transfer roller (33, 233) that transfers the developer image to the belt (23, 123, 223); wherein the image forming unit (11, 12, 13, 14) that forms the developer image has a developer; a cleaning member provided to clean the outer surface (23a, 123a, 223a) of the belt (23, 123, 223); a secondary transfer roller (33, 233) that secondarily transfers the developer image from the belt (23, 123, 223) to a recording medium; a detector (71, 271); and a controller that controls a rotation of the driving roller (20, 222) based on information of detection by the detector (71, 271), wherein the belt (23, 123, 223) comprising a detection target portion (61, 261) on an outer surface (23a, 123a, 223a) at an end of the belt (23, 123, 223) in a widthwise direction, wherein the detector (71, 271) detects the target portion (61, 261) of the belt (23, 123, 223); characterized in that when the belt (23, 123, 223) rotates, the detection target portion (61, 261) is configured to pass a cleaning member provided to clean the outer surface (23a, 123a, 223a) of the belt (23, 123, 223), the detection target portion (61, 261) includes an uneven pattern (61, 261) including at least one of: a plurality of grooves each of which extends in a rotating direction of the belt (23, 123), and a plurality of uneven points in the form of spots arranged in the rotating direction of the belt, the uneven pattern (61, 261) has a depth which is less than 2.3 times a mean particle diameter of the developer. an image forming unit (11, 12, 13, 14) that forms a developer image to be transferred to the belt (23, 123, 223); a transfer belt unit (10, 210) comprising a belt (23, 123, 223), a driving roller (20, 222) provided on an inner side of the belt (23, 123, 223) and driving the belt (23, 123, 223), a supporting roller (21, 22, 220, 221) provided on the inner side of the belt (23, 123, 223) and applying tension to the belt (23, 123, 223); a transfer roller (33, 233) that transfers the developer image to the belt (23, 123, 223); wherein the image forming unit (11, 12, 13, 14) that forms the developer image has a developer; a cleaning member provided to clean the outer surface (23a, 123a, 223a) of the belt (23, 123, 223); a secondary transfer roller (33, 233) that secondarily transfers the developer image from the belt (23, 123, 223) to a recording medium; a detector (71, 271); and a controller that controls a rotation of the driving roller (20, 222) based on information of detection by the detector (71, 271), wherein the belt (23, 123, 223) comprising a detection target portion (61, 261) on an outer surface (23a, 123a, 223a) at an end of the belt (23, 123, 223) in a widthwise direction, wherein the detector (71, 271) detects the target portion (61, 261) of the belt (23, 123, 223); characterized in that when the belt (23, 123, 223) rotates, the detection target portion (61, 261) is configured to pass a cleaning member provided to clean the outer surface (23a, 123a, 223a) of the belt (23, 123, 223), the detection target portion (61, 261) includes an uneven pattern (61, 261) including at least one of: a plurality of grooves each of which extends in a rotating direction of the belt (23, 123), and a plurality of uneven points in the form of spots arranged in the rotating direction of the belt, the uneven pattern (61, 261) has a depth which is less than 2.3 times a mean particle diameter of the developer. a plurality of grooves each of which extends in a rotating direction of the belt (23, 123), and a plurality of uneven points in the form of spots arranged in the rotating direction of the belt, the uneven pattern (61, 261) has a depth which is less than 2.3 times a mean particle diameter of the developer.