PURPOSE: To make it possible to improve the efficiency of the utilization of a frequency even on a system which uses a carrier relatively low in frequency stability, by amplitude-modulating a radio carrier by a time-division multiple system.

CONSTITUTION: As output signals from terminal apparatus 28 (for example, a digitized telephone set), signal pnlse string (e) and clock pulse string (d) are both supplied to primary modulator 26 and subcarrier 27 (b) is phase-converted (c), for example, and then supplied to main modulator 24, where a carrier (frequency fc = fu = an upward-circuit frequency) from main modulator 24 is amplitude-modulated (a) by its subcarrier, so that its output will be transmitted to center station 1 by way of output circuit 23, antenna common-use unit 22 and antenna 12. On the other hand, radio waves emitted at transmission frequency (fD) from center station 1 change into a code output by passing through antenna 12 of terminal station 2, common-use unit 22, 1st detector 29 detecting an amplitude-modulated wave, and 2nd detector 30 detecting a phase-modulated wave and then supplied to apparatus 28. The demodulation output of detector 29 composed of a peak detector circuit, etc., has relations only to amplitude and is under no influence of frequency stability.