PURPOSE: To attain binarization without use of a D/A converter by adopting a low sampling frequency for a reproduction signal.

CONSTITUTION: An analog reproduction signal is sampled and quantized by a period being a half of a shortest inter-code interval of the reproduction signal by an A/D converter 1, waveform interference is reduced by digital arithmetic operation by a DF 2, the resulting signal is binarized at a binarizing circuit 4 by using the quantized reproduction signal having a 1st sampling time and the quantized reproduction signal having a 2nd sampling time and coding a pulse signal at a rate of once per two samplings times. Moreover, the quantized reproduction signal levels at the 1st and 2nd sampling times are added, the sum is compared with a reference value to vary the conversion ratio tentatively to convert the reproduction signal into the binarized signal. A FIFO 5 outputs the binarized signal as a binarized code word in which the phase of the binarized codes is arranged.