To increase the speed of data transfer by continuously transmitting the entire division packets from a transmission side and returning response information including packet numbers which are requested collective re- transmission after a final packet is received at a reception side.

Transmission data are divided into packets 1-8, a packet number is given to each divided packet and data are transmitted by turning-on a carrier during the transmission of the divided packets 1-8. When the packets 1 and 3 do not arrive at the reception side in first-time transmission, the reception side sets the re-transmission packet numbers of the packets 1 and 3 in response information and transmits the response information. Since the re-transmission packet numbers of the packets 1 and 3 are set in the response information, the transmission side re-transmits the packets 1 and 3. The reception side can easily re-construct one packet from the divided packets since interruption from another station does not takes place at the reception side.