To enable communication of a high throughput by automatically selecting an optimal effective communication rate corresponding to a communication environment.

A master starts trial transmission from a packet of a high effective rate, activates a selection timer and repeats transmission and when a transmitting instruction is outputted by the set time of the selection timer on the basis of the regular data transmission start decided result, regular data are transmitted with a present packet class but when the transmitting instruction is not outputted by the set time of the selection timer, the trial transmission of the packet of the next high effective rate is started. At the same time, the selection timer is restarted and the trial transmission of the packet is repeated while lowering the effective rate step by step to the effective rate to output the transmitting instruction of the transmission start decided result. In the effective rate at which the transmitting instruction of the transmission start decided result is outputted, a packet class is decided and the regular data are transmitted/received.