An image forming apparatus comprising: an image forming unit that forms a developer image to be transferred to the belt ; a transfer belt unit comprising a belt , a driving roller provided on an inner side of the belt and driving the belt , a supporting roller provided on the inner side of the belt and applying tension to the belt ; a transfer roller that transfers the developer image to the belt ; wherein the image forming unit that forms the developer image has a developer; a cleaning member provided to clean the outer surface of the belt ; a secondary transfer roller that secondarily transfers the developer image from the belt to a recording medium; a detector ; and a controller that controls a rotation of the driving roller based on information of detection by the detector , wherein the belt comprising a detection target portion on an outer surface at an end of the belt in a widthwise direction, wherein the detector detects the target portion of the belt ; characterized in that when the belt rotates, the detection target portion is configured to pass a cleaning member provided to clean the outer surface of the belt , the detection target portion includes an uneven pattern including at least one of: a plurality of grooves each of which extends in a rotating direction of the belt , and a plurality of uneven points in the form of spots arranged in the rotating direction of the belt , the uneven pattern has a depth which is less than 2.3 times a mean particle diameter of the developer. an image forming unit that forms a developer image to be transferred to the belt ; a transfer belt unit comprising a belt , a driving roller provided on an inner side of the belt and driving the belt , a supporting roller provided on the inner side of the belt and applying tension to the belt ; a transfer roller that transfers the developer image to the belt ; wherein the image forming unit that forms the developer image has a developer; a cleaning member provided to clean the outer surface of the belt ; a secondary transfer roller that secondarily transfers the developer image from the belt to a recording medium; a detector ; and a controller that controls a rotation of the driving roller based on information of detection by the detector , wherein the belt comprising a detection target portion on an outer surface at an end of the belt in a widthwise direction, wherein the detector detects the target portion of the belt ; characterized in that when the belt rotates, the detection target portion is configured to pass a cleaning member provided to clean the outer surface of the belt , the detection target portion includes an uneven pattern including at least one of: a plurality of grooves each of which extends in a rotating direction of the belt , and a plurality of uneven points in the form of spots arranged in the rotating direction of the belt , the uneven pattern 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 , and a plurality of uneven points in the form of spots arranged in the rotating direction of the belt , the uneven pattern has a depth which is less than 2.3 times a mean particle diameter of the developer. <EOS>