PURPOSE: To stabilize the output frequency of a lower degree group signal received from a receiver by supplying continuously the stuff designating pulses at the input of the lower degree group signal of a transmitter and detecting the continuous stuff designating pulses via the receiver to control the frequency of the lower degree group signal to be outputted within a necessary standard.

CONSTITUTION: The input lower degree group signal subjected to a stuff process via a transmitter 1 is multiplexed together with a stuff designating pulse and transmitted to a receiver 3 as a higher degree group signal. The receiver 3 separates the higher degree group signal and outputs it as a lower degree group signal after a destuff process. The stuff designating pulses are continuously supplied when the input of the lower degree group signals of the transmitter 1 are interrupted. Then the receiver 3 detects the continuous stuff designating pulses and controls the frequency of the lower degree group signal to be outputted within the necessary standard. Consequently, the input interruption can be detected for the lower degree group input signals at the reception side and the output frequency is stabilized for the lower degree group signals received from the reception side.