An image forming apparatus includes a transport belt, a carriage, a reflection sensor, and a control unit. The transport belt transports a recording sheet in a sub-scanning direction. The carriage, mounting a recording head, reciprocally moves in a main scanning direction to record an image on the recording sheet. The reflection sensor, mounted on the carriage, receives light reflected from the transport belt to detect a leading edge of the recording sheet, and outputs a detection signal corresponding to the received light. The control unit controls a contamination check process. The control unit instructs the carriage to move to a given position over the transport belt and to drive the transport belt for one rotation while maintaining the carriage at the given position. The control unit determines whether contamination exists on the transport belt based on the detection signal output from the reflection sensor.