when a system is designed , a spare path corresponding to a working path set between adms is preliminarily determined , and spare path terminating point data is held together with working path terminating point data in a path terminating point control data memory 381 . line checking parts 41a and 41b , when detecting generation of a transmission line trouble in the working path , retrieve spare path terminating point data with reference to path terminating point control data 381 in a path terminating point controller 38 , and send out the spare path terminating point data together with working path terminating point data to the path switching part 372 . the path switching part 372 instructs an sts path switch 42 to switch the working path terminating point from the prevailing path terminating point over to the spare path terminating point , thus realizing bypassing of the working path via the spare path . after restoration of the trouble path , path switching to the initial state is made .