To generate highly accurate reference frequency signals by suppressing the influence of various kinds of frequency variation elements related to temperature.

A control part 10 observes the level of a synchronizing type control voltage signal with time, while 1PPS is inputted, and estimates and stores a control voltage signal for free-running. The control part 10 observes and stores the temperature of a voltage-controlled oscillator 14, on the basis of a temperature detection signal from a temperature sensor 16. When hold-over is detected, the control part 10 detects the temperature at the point of time (S101) and also calculates the time change rate of the temperature including the stored past temperature (S102). The control part 10: reads a coefficient for control correction value calculating set beforehand (S103); substitutes the detected temperature and the calculated time change rate of the temperature in an operation expression set by the coefficient; and calculates a control correction value (S104). The control part 10 corrects the control voltage signal for free-running, on the basis of the control correction value, and supplies it to the voltage-controlled oscillator 14.

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