PURPOSE: To make it possible to efficiently execute the interruption processing at a high speed, by rewriting a status display table of the master station automatically against a status change of the slave station so that the responsiveness to an interruption request of the slave station can be elevated.

CONSTITUTION: The master station 2 sets the polling flags of all the slave stations to "1" at first, subsequently sets the number N of continuous polling, and executes the polling to all the slave stations. And the master station 2 which has received a response from the slave station sets a flag indicating that the slave station is being connected, to the slave station address related to the polling (status display of master station) table (RAM7). In this case, if the slave station which has received the polling does not send back a response, the polling table is reset (flag is set to "0" instead of "1"). And when the polling to all the slave stations 3a∼3n has been finished, this time the master station 2 executes the polling to only the slave station whose flag is "1" in accordance with the polling table.