The encryption key distribution system includes a node A, a node B, and an authentication server S. Upon receiving a first nonce created by the node A and a second nonce created by the node B, the authentication server S creates a session key. The authentication server S sends a first message authentication code value and a first encryption message to the node A. The first encryption message is created by encrypting the first nonce, the session key, and first additional information with a first secret key. The authentication server S sends a second message authentication code value and a second encryption message to the node B. The second encryption message is created by encrypting the second nonce, the session key, and second additional information with a second secret key. The node A decrypts the first encryption message with the first secret key, thereby obtaining the session key and the first additional information. Further, the node A authenticates the session key by use of the first message authentication code value. The node B decrypts the second encryption message with the second secret key, thereby obtaining the session key and the second additional information. Further, the node B authenticates the session key by use of the second message authentication code value.