PURPOSE: To generate FSK signals high in frequency stability and low in inter- code interference.

CONSTITUTION: A phase locked loop 1 is provided with a voltage controlled oscillation circuit(VCO) 11 to send an oscillation signal S1 at a frequency in the case of spacing, and a phase-locked loop 2 is provided with a VCO 21 to send an oscillation signal S2 at a frequency in the case of marking. A reference signal generation circuit 3 generates a reference signal Ss to be supplied to the phase-locked loop. A switch circuit 4 selects either a controlled voltage V1 of the VCO 11 or a controlled voltage V2 of the VCO 21. A low-pass filter 5 is composed of a circuit which group delay characteristic is flat. A switch circuit 6 selects either the output of the switch circuit 4 or the output of the low-pass filter 5. A VCO 7 is controlled by the output selected by the switch circuit 6 and sends an oscillation signal S3. A frequency divider circuit 8 frequency divides the oscillation signal S3 and prepares the prescribed FSK signal. A switch control circuit 10 controls the respective switch circuits based on input data Di.