An image forming apparatus may include a conveyor member, an optical sensor, a drive portion, and a controller. The drive portion performs a rotation for detection to rotate the conveyor member for the mark detection and a rotation for nondetection to rotate the conveyor member for a purpose other than the mark detection. The controller obtains an amount of reflected light received by the optical sensor to determine an adjustment value of sensitivity of the optical sensor based on the amount of received reflected light while the rotation for nondetection is performed. The controller also detects the mark with the sensitivity adjusted based on the determined adjustment value while the rotation for detection is performed.