A communication system includes a plurality of communication terminals that form a communication group and a node device that carries out an authentication process on each of the communication terminals. The node device derives first keys unique to the respective communication terminals by using information shared between the node device and each communication terminal through the authentication process, derives a second key common to the communication group, calculates an exclusive OR between each first key and the second key, and transmits respective XOR values obtained through the calculation to the respective communication terminals. Each communication terminal reproduces the second key by calculating an exclusive OR of between the first key unique to the own communication terminal derived by using the information and the XOR value received from the node device. Thus, the keys used in group communication are managed more securely.