To change a modulation system, etc., for every packet to attain the communication by preparing plural modulation means of different transmission speeds, transfers the digital signal sent from a signal source to one of those modulation means based on a prescribed reference, and transmitting the packet that is modulated by every modulation system only.

If only an approval signal is transmitted, a mode decision part 2 sends the received digital signal produced by a signal source 1 to a slow modulation part 6 which performs the frequency shift keying modulation via a D/A converter 3 in a slow mode and stops the operation of a fast modulation part 7. When a large quantity of information data are transmitted, the part 2 sends the data to the part 7 via the D/A converters 4 and 5 in a fast mode and stops the operation of the part 6. Then the frequency of the signal that is modulated by the part 6 or 7 is increased up the radio frequency by the mixers 9 and 12, the amplifiers 11 and 14, and a BPF 15 and transmitted via an antenna 16.