PURPOSE: To always read a read mark without fail by passing a signal from the light receiving part of a phototransistor through a differentiation circuit, extracting the edge of the read mark and shaping the waveforms of a set pulse, which is made from an extracting signal, and a reset pulse to a digital waveform by a flip-flop circuit.

CONSTITUTION: Reflected light on a bar code is image-formed on a phototransistor 11 by a lens and for an electric signal from the phototransistor 11, the edge from white to black or from black to white of the bar is detected by a differentiation circuit 12. Based on this edge detecting signal, the pulse and reset pulse are outputted and these both pulses are inputted to a flip-flop circuit 15. Then, the digital waveform for digital processing is taken out and digital processing is executed to this digital waveform. After that, the information of the bar code are read. Thus, even when the scan speed of a reader is low and the width of the read mark is wide, the information of the read mark can be read without fail.