PURPOSE: To improve the throughput of event transmission by detecting the omission of a synchronous data frame while deciding whether a running number added to the synchronous data frame is specified one or not at a reception side station.

CONSTITUTION: A station that receives a synchronous data frame 20 transfers it to a reception buffer memory 4b. A transmission source address is recognized from a frame header 10, the value of a running number table 3c corresponding to this address is compared with the running number on the synchronous data frame 20, when those running numbers are the same, it is judged that the synchronous data frame is completely received the last time, and reception processing is not performed. When the running number on the received synchronous data frame is larger just for '1', it is stored in a cyclic memory 6, and the transmission source address and running number of the synchronous data frame 20 are stored in a media access part 3c. In the other case, the omission of the synchronous data frame is judged, and the transmission source address and the running number are stored in a retransmission request transmission table 3d.