A key distribution system includes a representative user terminal 2p, a server apparatus 3, and an (n+1)-th user terminal 2n+1. The representative user terminal 2p uses a public key for the (n+1)-th user terminal 2n+1 and information for identifying the (n+1)-th user terminal 2n+1 to encrypt key information with a predetermined encryption function in Certificate-less Encryption to obtain ciphertext. The server apparatus 3 sends the ciphertext to the (n+1)-th user terminal 2n+1 when the (n+1)-th user terminal 2n+1 is added. The (n+1)-th user terminal 2n+1 uses a complete secret key for the (n+1)-th user terminal 2n+1 and the information for identifying the (n+1)-th user terminal 2n+1 to decrypt the ciphertext with a predetermined decryption function to obtain the key information.