In a carrier relay system in which protective relay apparatus are disposed in terminal stations respectively and interchange information between the terminal stations via a signal transmission line for detecting a trouble occurred in power transmission lines on the basis of the exchanged information, specific information K is inserted in a predetermined position of a signal transmitted from the first terminal station to the second terminal station, and the specific information K is inserted intact in another predetermined position of a signal returned from the terminal station to the terminal station. In the terminal station, the specific information K inserted in the signal transmitted therefrom is compared with the specific information K inserted in the signal returned from the terminal station for deciding the presence or absence of a trouble occurred in the power transmission lines. The signal transmission line extending between the terminal stations includes two transmission channels, and circuit breakers are tripped when a fault has occurred in one or both of the signal transmission channels and a trouble detection relay has operated.