PURPOSE: To obtain a phase-synchronous oscillator which has high frequency purity by synthesizing the frequencies of the output of a voltage-controlled oscillator and that of a local oscillator, and by increasing the output variations of the voltage- controlled oscillator by multiplying the resulting frequency.

CONSTITUTION: The output frequency of a voltage-controlled oscillator 11 is mixed by a frequency converter 12 with the frequency of the local oscillation signal of a local oscillator 13. The output of the frequency converter 12 has its frequency multiplied by M by a multiplier 19, whose output is divided by N at a variable frequency divider 14. The output phase of the variable frequency divider 14 is compared with the phase of a reference signal by a phase comparator 16, whose output is supplied as a control signal to the voltage-controlled oscillator 11 by way of a low-pass filter 17. Therefore, the output variations of the voltage-control oscillator 11 are increased by M times by the multiplier 19, so that when the phase comparator 16 makes the comparison, the variations are detected sufficiently.