To acquire an in-device operating frequency of high precision irrelevantly to the precision of an in-device oscillator by correcting the output frequency of a digital synthesizer according to the deviation of the output frequency of the in-device oscillator from a reference frequency.

A counter 23 counts the oscillation frequency of an internal oscillator 22 with one-second pulses from a GPS receiver 21 as a gate signal to find the deviation showing how much the oscillation frequency of the internal oscillator 22 deviates from the reference frequency. A CPU 27 finds an addition/ subtraction value supplied to one input of an adder subtracted 25 according to the counting result of the counter 23 and determines the contents of a frequency sending-out instruction supplied to a memory 24. The memory 24 supplies frequency data corresponding to the frequency data sending-out instruction to the input of the adder subtracted 25. The adder subtracted 25 generates and outputs correction frequency data to the digital synthesizer 26.