An image forming apparatus includes a sheet feeder, a sheet conveyor, a line head, an image sensor, a moving mechanism, and a controller. The image sensor has a reading width in the main scanning direction smaller than main scanning direction widths of a largest printable size paper sheet. In setting for using a large width sheet, a controller controls a moving mechanism to move the image sensor toward one side so that a one side edge of the large width sheet can be read. Based on read image data obtained by reading, the controller recognizes a deviation direction and a deviation amount of a position of the conveyed large width sheet in the main scanning direction.