PURPOSE: To attain communication and control simply, by designating a non- modulation area between frame signals obtained from the speed conversion of digital signals of frame constitution as an area for service signal transmission.

CONSTITUTION: A speed conversion circuit SPC speeds up a PCM signal so as to decrease the time for one frame and a modulation circuit MD1 modulates a carrier wave. Service signals SVS1, SVS2 are synthesized at a multiplex circuit MDX via the conversion circuit SPC, and the output of the MD1 is modulated at a modulation circuit MD2. In controlling a changeover switch SW with the timing pulse for synchronizing signal from an oscillation circuit SP, a signal of non- carrier is obtained and transmitted from an antenna AT via a transmitter TR. The service signal SVS is inserted to a vacant part TSP between adjacent frames produced with the conversion of bit rate.