There are disclosed a transmission device and a reception device having a high secrecy making it difficult to wire tap by a third device. The transmission device includes a modulation section which can be modulated by a plurality of modulation methods and a transmission section which can transmit a radio signal by using a plurality of carrier frequencies. Moreover, the reception device includes a demodulation device which can demodulate by using a plurality of demodulation methods and a reception section which can receive a radio signal by using a plurality of carrier frequencies. The transmission communication method or the reception communication method is changed as the time elapses by changing the combination of the modulation method or demodulation method and the carrier frequency, thereby transmitting or receiving data.