PURPOSE: To absorb the clock variation generated when the clock is switched by using a phaseclocked oscillating circuit to a clock supply circuit.

CONSTITUTION: The phase are different between clocks generated from two clock sources, and the 1st clock source 400 is changed to the 2nd clock source 401. In this case, the pulse width of an output (c) of a clock phase comparator 404 has variation. Then the output waveform (c) is compared with the output of a voltage control oscillator VCO406 through a clock phase comparator 405. Based on the result of this comparison, the output (d) of the VCO406 synchronizes gradually with the clock (b) of the source 401. In this case, a big variation of pulse width can be absorbed like the waveform (c) obtained when the clock of the circuit 404 is switched.