PURPOSE: To avoid bit conversion mistake, by feeding back a whisker waveform including in a signal consisting of NRZ codes after conversion to a delay time adjusting circuit provided at a clock extracting section.

CONSTITUTION: After a whisker waveform included in a signal of NRZ codes extracted from an output G' of an exclusive OR circuit 16 via an AND circuit 20 is amplified at an amplifier 21, it is converted into an analog signal L' with a low- pass filter 22. After this analog signal L' is converted into a digital signal by an A/D converter 23 and a decoder 24, it is given to a counter 13 operating as a delay time adjusting circuit as a frequency dividing ratio (count information) to control the number of counts so that the width of the whisker waveform included in the signal of the NRZ code can be minimized for the counter 13.