To simplify a circuit, shorten a leading time, and achieve stable operation by preliminarily applying an outside control voltage to be oscillated in the pull-in range of phase comparator means from a temperature/voltage converting means to a VCO.

Input data and a signal outputted from a VCO 14 are compared by a phase comparator 11 so that deviation from the optimal phase can be detected, and a control voltage corresponding to the result is outputted by the phase comparator 11. The output signal of the phase comparator 11 is smoothed by a loop filter 12, and applied as the control voltage of a VCO 14, and the oscillator frequencies and phase of the VCO 14 are held so as to be optimal. A temperature/voltage converting part 13 using a temperature sensor is provided so that the oscillation frequencies of the VCO 14 can be prevented from extensively fluctuating, and an outside control voltage to be oscillated in the pull-in range of the phase comparator 11 is preliminarily applied to the VCO 14, and optimally changed according to the surrounding temperature so that a constantly stable PLL operation can be attained.