PURPOSE: To convert a precise digital waveform signal even if an AC voltage waveform signal outputted from a reader is any signal by regarding the AC voltage having the waveform obtained by removing a point part of the AC voltage waveform signal as a reference voltage.

CONSTITUTION: The bar code of a label 1 is read out by a hand scanner 2, amplified and outputted to a reference voltage variable circuit 8 through a buffer amplifier circuit 7. A switching element 9 in the reference voltage variable circuit 8 connected when forward and backward voltages exceed a fixed value and disconnected at a value less than the fixed value, so that the reference voltage to be impressed to a comparator 4 has a waveform of which point part is removed. Since the reference voltage is changed so as to follow an AC voltage waveform signal, the analog AC voltage waveform signal can be converted into a precise digital waveform signal even if any recording information is read out.