PURPOSE: To generate a sampling signal giving the appropriate timing of sampling by comparing a time which a sampling timing measuring means has measured with a prescribed sampling timing set point.

CONSTITUTION: Whenever there is a change in a serial data signal from '0' to '1' or from '1' to '0', the sampling timing measuring means 12 measures the time from when data is changed up to the timing of sampling. Then, a sampling timing decision means 14 and a variable frequency signal generation means 10 operate in such way that the time becomes equal to the prescribed sampling timing set point, namely, the period of 1/2 of a bit rate. Thus, the sampling signal giving the appropriate timing of sampling can be generated.