The present invention is provided with one main station and a plurality of substations. After transmitting data addressed to all substations, the main station transmits a message addressed to all substations to query whether reception was normal. When unable to receive data normally, the substations transmit a response message to the query message, but do not transmit a response message when data reception was normal. The time for confirming that transmission was normal or abnormal can be shortened by the main station transmitting data addressed to all substations only one time, and by the substations transmitting response messages only in the case of abnormal reception.