Abstract: In accordance with a network protocol having an IP layer, a MAC layer and a physical layer, an MPDU formed in the physical layer is transmitted through a LAN by radio transmission between a transmitting-side transmission terminal and a receiving-side transmission terminal among a large number of transmission terminals. When a part of the transmission terminals, using the extended network protocol where an extended MPDU having a data amount larger than a data amount standardized in the network protocol can be formed, is a transmitting-side transmission terminal, if it is determined through the LAN that a receiving-side transmission terminal is applicable to the extended network protocol, the transmitting-side transmission terminal forms an extended MPDU in the physical layer in accordance with the extended network protocol, and transmits the extended MPDU through the LAN to the receiving-side transmission terminal by radio transmission.