A communication system, which performs communication using a transmission packet encrypted by an IP-SEC encrypting method, includes a first encrypting circuit that encrypts a transmission packet by an IP-SEC encrypting method, a second encrypting circuit that encrypt header data to be used to decode the transmission packet encrypted by the first encrypting circuit, and a transmitting circuit that transmit the transmission packet whose header is encrypted by the second encrypting circuit. The communication system further includes a first decoding circuit that decode the authentication data of the reception packet using information to be used to decode the authentication data recorded in the IP-SEC header of the transmission packet and a second decoding circuit that decodes the reception packet using the authentication data decoded by the first decoding circuit.