PURPOSE: To reduce the network cost by devising the frequency precision of a subordinate station not to be limited by frequency accuracy of a station in the self-running state even when a station on a clock path is in the self- running state so as to realize a highly qualified synchronization network.

CONSTITUTION: An addition means 10 of a host station A on a clock path 1 of an active system adds clock path monitor information to a synchronization digital hierarchy multiplex signal and sends the result. A detection means 11, of a subordinate station C detects the clock monitor information added to the digital hierarchy multiplex signal from the host station A and a changeover means 12 is used to select the standby system clock path 2 based on the result of detection of the detection means 111.