PURPOSE: To decrease frequency handled at a frequency converting part, to expand a frequency pull-in scope and to suppress frequency fluctuation by using frequency fluctuation information obtained in a carrier wave generating part of the four-phase DPSK (QPSK) signal, as frequency control information for the voltage control oscillator of the second frequency converting part.

CONSTITUTION: In a QPSK demodulator 34, a QPSK signal is inputted into phase detecting devices 35A and 35B, a signal for direct phase detection of a voltage control oscillator 38 and a signal for phase detection, which causes the signal to shift to 90° by a phase shifter 39 are used, and a detecting output of the base band is obtained by respective phase detecting devices 35A and 35B. Respective detecting outputs are inputted to a multiplying device 36 for analog multiplication, the multiplying output is inputted into a filter 37 and made average and the output of the filter 37 is used as a control signal through a level shifting circuit 33 to a voltage control oscillator 32 of the second frequency converting part 30. As this result, in the second frequency converting part 30, four multiplying circuits and a reference oscillator are not necessary to be used and the frequency pull-in scope can be expanded.