The automatic document feeder includes a multi-feed detection sensor unit, a sensor control unit, and a control board on which the sensor control unit is mounted. The multi-feed detection sensor unit includes a transmission board on which a transmission sensor and a detection circuit of a drive current of the transmission sensor are mounted, a reception board on which a reception sensor and an amplifier circuit of a reception signal of the reception sensor are mounted, and a relay board on which a drive circuit of the transmission sensor is mounted. The sensor control unit stores first fault information indicating that the relay board is disconnected if the relay board is disconnected, stores second fault information indicating that the reception board is at fault if an output value of the amplifier circuit is equal to or less than a first threshold, stores third fault information indicating that the transmission board is at fault if the detection circuit does not detect the drive current, and stores fourth fault information indicating that the transmission sensor or the reception sensor is damaged if the output value of the amplifier circuit is equal to or less than a second threshold. If the fault information is stored, the sensor control unit transmits, to an image forming apparatus, an instruction signal for causing the fault information to be displayed.