PURPOSE: To prevent such a case where the communication is discontinued due to the abnormality of a network synchronizing clock by shortening the time to detect abnormality and then the network synchronizing clock source is replaced with another one.

CONSTITUTION: The line concentration clock lines 3-1 and 3-2 are used for a switchboard connected to a wide network or for a network synchronizing clock distributor included in a multiplexer. Then the circuit interface parts 1-1-1-3 transmit the network synchronizing clocks to the lines 3-1 and 3-2 independently of each other. A clock generating part 2 checks and selects a normal clock.