PURPOSE: To simplify circuit constitution and to stabilize operation by executing frequency shift modulation with a digital operation processing.

CONSTITUTION: When it is defined that the phase difference of a sampling frequency corresponding to modulation frequency is Δθ and a current phase is θ, a sine wave signal at the timing of the succeeding sampling period is sin(θ+Δθ,)=sinθ.cosΔθ+cosθ.sinΔθ, so that the preceding operation result and the phase difference Δθ are read out from a memory 6, the 1st and 2nd terms of the right side of above-mentioned equation are multiplied each other by a multiplier 5, the multiplied result is added by an adder 4, the added value is stored in the memory 6 so as to be used for the succeeding calculation and also applied to a D/A converter 2 so as to be converted into an analog signal. Since the operation is executed by using a phase difference corresponding to the preceding computed result and the succeeding modulation frequency at change points '1', '0' of input data, no distance is generated in the amplitude and the phase. Consequently, the circuit size can be reduced and the jump of the amplitude and the phase at the time of shifting the frequency can be suppressed.