PURPOSE: To prevent the momentary disconnection of line, by providing the phase synchronous circuit and multiplier obtaining self running frequency stably with the transmission section of existing and spare unit and providing the highly stable reference oscillator in commonly with existing and spare unit.

CONSTITUTION: The phase synchronous circuit which can produce stable self running frequency including the voltage controlled crystal oscillators VCXOa,VCXOb and the multipliers Ma,Mb multiplied to the carrier frequency such as microwave band are respectively provided with the existing and spare transmission section Ta and Tb, and the highly stable reference oscillator OSC is commonly provided to the Ta and Tb. Accordingly, the transmission frequency is not changed due to the selection of existing and spare unit. Further, even if the highly stable reference oscillator OSC is in failure, since the operation of transmission can be kept with the self running frequency of the phase synchronism circuit, no momentary circuit disconnection can be taken place.