PURPOSE: To read out bar code symbols by accurately detecting signal transition points by a differentiating circuit.

CONSTITUTION: A scan is made across a bar code symbol and a light spot for scanning, which is reflected by the bar code symbol, is detected to generate an analog signal representing it. Then, the primary derivative of the analog signal is found and compared with a primary derivative signal which is delayed by a delay circuit 18 to detect the transition of the signal. A false transition gating circuit means 22 which changes a state only when a transition signal larger than a given threshold value is detected changes the state of an output signal only when changing the state by discriminating the false transition signal.