A monochrome image forming apparatus includes an image forming unit that forms a black image, a motor that drives the image forming unit, a control circuit that controls the motor, a first connector to which the motor is connected via a control line, a second connector to which a control target is not connected, and a substrate on which the control circuit, the first connector, and the second connector are arranged. A predetermined mark is formed on the substrate through silk screen printing. A distance between the predetermined mark and the second connector is shorter than a distance between the predetermined mark and the first connector. If the substrate is disposed in a multi-color image forming apparatus configured to form a color image, a control target for the multi-color image forming apparatus is connected to the second connector.