PURPOSE: To prevent the abnormalities of the circuit actuation by detecting the break of clocks of two signal systems having an extremely small difference of clock frequency for transfer of digital data signals and switching the clock source to another system.

CONSTITUTION: The break of the input data 1 given from one of both digital signal systems is detected 4 and an input break alarm signal 5 is sent to a clock source selecting circuit 22. The circuit 22 selects the digital signal of one of both signal systems and a clock signal 10 extracted from the other digital signal system and delivers them as a clock component signal 23. Then a clock extracting circuit 2 extracts a clock. This extracted clock signal 3 is supplied to a frame extracting circuit 6, an input buffer memory 9, an output buffer memory 13 and a transmission frame producing circuit 16 respectively. This can prevent the abnormalities of the circuit actuation produced when a clock is stopped between two signal systems which have the clock frequencies approximate to each other and transfer data via buffers 9 and 13.