PURPOSE: To improve FSK modulation, by selecting and controlling a sinusoidal wave which forms the mark signal frequency of positive and negative phase, and a space signal frequency of both phases, so that the phase at the end of the signal frequency can be made continuous.

CONSTITUTION: The output of an oscillator 1 is frequency-divided 2 into a specified mark signal, shaped into a sinusoidal wave at a band spread filter 3, amplified directly or inversely 5, and given to an analog multiplexer 4. Similarly, the space signal is inputted to the multiplexer 4. On the other hand, the output from the oscillator 1 is made to a clock signal corresponding to a data transmission speed with the frequency-division 9, output is made from an output terminal 10 to the data device, and the data is inputted to a control signal forming circuit 12 from an input terminal 11. Thus, the multiplexer can be switched so that the phases at the end of the mark signal frequency and the space signal frequency can be made continuous.