An image forming apparatus includes a belt, a detector, a determining section, a skew correcting section, and a storage section. The belt is stretched around rollers and provided with an index. The detector provides an output value. The determining section determines the index that is passing through the detection region on the basis of a change in the output value, the change exceeding a threshold. The skew correcting section performs control relating to skew correction of inclining the roller and correcting the position of the belt. The storage section stores the threshold having a value between the measured maximum amount of change in the output value when the skew correcting section inclines the roller at the maximum angle and the amount of change in the output value resulting from the index when the index arrives at or passes through the detector.