PURPOSE: To surely sent a reply signal to a master station even when a gate control signal is sent from plural slave stations due to malfunction by converting a gate control signal from each slave station into a priority gate control signal with a signal conversion means such as a ROM thereby controlling the switching of a gate.

CONSTITUTION: A slave station such as 2b detecting the designation of its own station generates a reply signal 6b and a gate control signal 7b and sends them to a signal selector 3. Through the malfunction of a slave station 2c, when slave stations 2b, 2c send reply signals 6b, 6c and gate control signals 7b, 7c to a signal selector 3 with respect to a polling signal 5 from a master station 1, an address '011' of a ROM 8 is accessed and its data '010' is read. Thus, since only a priority gate control signal 9b among priority gate control signals 9a-9c goes to logical 1, only the gate 4b is opened and the reply signal 6b from the slave station 2b is transferred to the master station 1.