An image forming apparatus includes a movable image-bearing member, a sensor opposed to the image-bearing member, a shutter member arranged between the image-bearing member and the sensor and capable of moving between an open position and a closed position, a rotatable cam configured to move the shutter member, and an execution portion configured to selectively execute a first mode in which the shutter member is opened or closed by rotating the cam within a first phase range and a second mode in which the shutter member is vibrated by rotating the cam at least through a second phase range different from the first phase range.