PURPOSE: To eliminate difficulties in uniform processing due the handling of the halfway number of bits by dividing plural groups into plural tracks, and performing system polling, track by track.

CONSTITUTION: A receiver belonging to a group which responds to a receiver in system polling for transmitters is called, and the timing of response to the receiver is made different by the transmitters in the group; and a transmitter which has changes in state responds to the receiver at response timing assigned to itself. Further, when a transmitter responds to the receiver in point polling for the transmitters, transmitters having responded to point polling for the transmitters among the transmitters in the group are called individually and specific information on the transmitters is gathered. Namely, the groups are divided into the tracks in this case, track information is included in a command, and the system polling is carried out, track by track.