An image forming apparatus includes an image forming mechanism, a sheet conveyance mechanism, and a sheet detector. The image forming mechanism forms an image and transfers it onto the sheet conveyed by the sheet conveyance mechanism. The sheet detector is arranged upstream from an image transfer region and configured to detect the sheet. The sheet detector includes a light source, a plurality of photoreceptors, and a controller. The plurality of photoreceptors is arranged in line in a main-scanning direction and includes a first photoreceptor group and a second photoreceptor group. The first photoreceptor group changes output voltages when overridden by the side edge of the sheet. The second photoreceptor group includes an opposite end of the plurality of photoreceptors being out of the side edge of the sheet. The controller is configured to adjust a light emission amount in accordance with characteristics of the sheet.