PURPOSE: To stabilize a transmission frequency of a slave station, by inserting a non-modulation carrier to an FSK signal directed from a master to slave station, picking up this non-modulation carrier at the transmission of the slave station and taking the same as a reference frequency.

CONSTITUTION: A master station uses an FSK modulation system and transmits signals in time division to a plurality of slave stations. In this case, a non-modulation carrier is inserted to the head or tail of a frame of the FSK signal from the master station. When the slave station inputs the FSK signal from the master station, the non-modulation carrier is detected at a frequency discriminator 13, this is inptted to a controller 14 and controls a modulator 11 with an AFC circuit as a reference frequency. Thus, the tansmission frequency to the FSK signal transmitted from the slave station is made stable for transmssion by taking this reference frequency as the base.