To prevent higher-harmonic waves, generated from a step recovery diode wherefrom used specified harmonic waves are excluded removed from leaking, from the diode to the output side of the voltage-controlled oscillator of an analog phase-locked oscillator.

The analog phase-locked oscillator has a voltage-controlled oscillator 5, whose oscillation frequency is controlled by a controlling voltage; a reference oscillator 1; and a sampling phase detector 3, whereto the oscillation signal of the voltage-controlled oscillator 5 and the reference oscillation signal of the reference oscillator 1 are inputted, and wherefrom the error voltage, obtained from the phase difference between the oscillation signal of the voltage- controlled oscillator and harmonic waves of the reference oscillation signal are inputted to the voltage-controlled oscillator 5 as its controlling voltage. Further, a non-reciprocal circuit element 6 is interposed between the voltage- controlled oscillator 5 and the sampling phase detector 3 so as to be input via the non-reciprocal circuit element 6 to the sampling phase detector 3 the oscillation signal outputted from the voltage-controlled oscillator 5.