PURPOSE: To sufficiently suppress shift of frequencies of each slave station, by reproducing carrier waves in signals from a master station received commonly with each slave station and synchronizing a transmission frequency to the carrier frequency.

CONSTITUTION: A communication device of a slave station is provided with a phase synchronizing circuit consisting of a voltage controlled oscillator 24, phase comparator 25, and loop filter 26, in place of a carrier wave oscillator. Further, a demodulator 19' picks up a carrier wave (p) reproduced from a reproducing circuit of carrier wave and gives it to the phase comparator 25. The carrier wave (p) obtained from it is given to the comparator together with an output (j) of the oscillator 24 to synchronize the oscillator 24 via the filter 26.