Abstract: Receiving section 102 receives security data. Propagation estimating section 103 estimates a propagation environment based on a reference signal transmitted from a communicating party. Demodulation section 104 demodulates the security data. Coding section 105 encodes information of estimated propagation environment to generate confidential information corresponding to the propagation environment. Buffer 106 temporarily stores the encoded confidential information. Decoding section 107 interprets the security data encrypted using an encryption key input from demodulation section 104, using encryption key information input from buffer 106, to decode, and acquires the security data. It is thereby possible to ensure high security without changing the communication system greatly.