A highly reliable and high-speed data transmission is made by using an FH or MFSK mode as suitably selected according to the usage of channels. When a reception signal is entered into a receiver through a transmission line, (i) a signal processing unit supplies the spectrum intensity values of carrier frequencies, (ii) a channel detection unit controls the phase of a time slot based on the spectrum intensity values, selects the MFSK or FH modulation mode, and supplies reception code data corresponding to detected carrier frequencies, and (iii) a decoder supplies reception information data based on the reception code data. When transmission information data are entered into a transmitter, (i) a coding unit supplies, according to the selected modulation mode, transmission code data based on the transmission information data, (ii) a channel generation unit supplies, based on the transmission code data, carrier frequencies to be used and (iii) a waveform generation unit supplies a transmission signal to a transmission line.