PURPOSE: To attain address participation optionally by providing a polling check code to a command data frame to attain polling only to a required terminal device automatically and use a common interruption address.

CONSTITUTION: A polling check signal (CK) is assigned to a command data frame. After application of power, a control center 1 adds a CK code and applies sequentially polling and each terminal device outputs a response signal 8 at own address transission. The control center 1 stores a response address and outputs a response address group and an interruption address at the next polling. The devices of addreses 0, 2 have a response and the polling is applied from the next time and the device of an address 1 gives no response and is not an object of the next polling. The said interruption address is utilized when the terminal device of the address participates in the polling during the execution of polling.