PURPOSE: To provide a clock synchronizing device capable of supplying a sampling clock synchronized with a wide range clock applied from the external and reduced at its jitter to a D/A converter, an A/D converter, or the like.

CONSTITUTION: This clock synchronizing device is provided with a highly stable voltage control oscillator 1, a reference clock phase comparator 2, a voltage control oscillator 3, an external clock phase comparator 6, an n-ary counters 5, an up/down counter 7 for controlling the frequency dividing ratio (n) of the counter 5. The frequency dividing ratio of the counter 5 is changed based on a compared result between the frequency of an external clock and an output signal from the oscillator 3, the frequency of an output signal from the counter 5 is compared with that of an output signal from the oscillator 1 by the comparator 2 and an output signal from the comparator 2 is used as the control voltage of the oscillator 3 to provide a clock with small jitter.