An oscillator includes phase frequency detectors, each detecting the phase difference between two input signals (output signal and external reference signal) and outputting a control command signal for controlling the output signal to achieve a desired frequency on the basis of the phase difference. A plurality of ICs, each including a phase frequency detector, frequency dividers, a charge pump, and a lock detection circuit, is operated in parallel. A composite control command signal generated by combining outputs of the phase frequency detectors is output via a loop filter to a voltage-controlled oscillator. Whether phase noise is reduced sufficiently is determined on the basis of detection results by an amplitude detection circuit for detecting the amplitude of an AC component of the composite control command signal and the lock detection circuits. The phase frequency detectors are repeatedly reactivated until the phase noise is reduced sufficiently.