An error detecting unit of a phase-locked oscillator evaluates difference between a reference phase error signal output from a phase detector and a phase error signal actually output from the phase detector when a reference frequency modulation signal is output from a voltage-controlled oscillator and further detects a frequency error of the frequency modulation signal from the voltage-controlled oscillator based on a rate of change of the difference. A correction unit of the phase-locked oscillator calculates an average value of the frequency error in a predetermined section of the frequency modulation signal and corrects center frequency of the frequency modulation signal by correcting the average value to be zero, and changes the rate of change of control voltage per control step based on comparison between at least two frequency errors in one cycle of the frequency modulation signal. Thus frequency shift of the frequency modulation signal is corrected.