A communication device (3-1, 3-2) comprising: a first subscriber identification unit (100-1, 100-2) that is adapted to store first subscriber identification unit identification information associated with first user identification information; and a communication unit that is adapted to communicate with another communication device (3-1, 3-2), the other communication device (3-1, 3-2) comprising a second subscriber identification unit (100-1, 100-2) that stores second subscriber identification unit identification information associated with second user identification information, wherein the first subscriber identification unit (100-1, 100-2) comprises: a certification authority key memory unit that is adapted to record a pair of a certification authority public key certificate and a certification authority secret key, the pair being shared with the second subscriber identification unit (100-1, 100-2), the first subscriber identification unit (100-1, 100-2) further comprises: a public key certificate generation unit that is adapted to generate a first public key certificate comprising a first public key; the first subscriber identification unit identification information or the first user identification information; and an electric key signature of the first public key, the electric key signature being encrypted data resulting from an encryption, with the certification authority secret key, of data including the first public key and the first subscriber identification unit identification information or the first user identification information, the communication unit is adapted to transmit to the other communication device (3-1, 3-2), the first public key certificate, and the first public key certificate is generated in a secure random access memory of a subscriber identity module of the communication device (3-1, 3-2). a first subscriber identification unit (100-1, 100-2) that is adapted to store first subscriber identification unit identification information associated with first user identification information; and a communication unit that is adapted to communicate with another communication device (3-1, 3-2), the other communication device (3-1, 3-2) comprising a second subscriber identification unit (100-1, 100-2) that stores second subscriber identification unit identification information associated with second user identification information, wherein the first subscriber identification unit (100-1, 100-2) comprises: a certification authority key memory unit that is adapted to record a pair of a certification authority public key certificate and a certification authority secret key, the pair being shared with the second subscriber identification unit (100-1, 100-2), the first subscriber identification unit (100-1, 100-2) further comprises: a public key certificate generation unit that is adapted to generate a first public key certificate comprising a first public key; the first subscriber identification unit identification information or the first user identification information; and an electric key signature of the first public key, the electric key signature being encrypted data resulting from an encryption, with the certification authority secret key, of data including the first public key and the first subscriber identification unit identification information or the first user identification information, the communication unit is adapted to transmit to the other communication device (3-1, 3-2), the first public key certificate, and the first public key certificate is generated in a secure random access memory of a subscriber identity module of the communication device (3-1, 3-2).