PURPOSE: To obtain miniaturization and light weight and to improve operability by allowing a party-line telephone set interface and an exchange interface in a signal repeater which is used when an emergency circuit or a temporary circuit is installed, and constituting the line of the same two devices.

CONSTITUTION: For a signal relay system, the route of party-line telephone sets 1 and 2 - a signal repeater 3 - a transmitter 4 - a transmitter 4' - a signal repeater 3' - a subscriber's switch 5 - an exchange 6 is formed when a party-line telephone subscriber's line is installed between the party-line telephone sets 1 and 2, and the exchange 6. Both party-line telephone set interface and exchange interface are allowed for outgoing from the party-line telephone set 2, incoming to the party-line telephone set 1, and incoming operation to the party-line telephone set 2, and the circuit constitution is executed by the same two devices.