A data communicating method is disclosed for use with a user terminal, the method preventing both members of a given group including the terminal and nonmembers from intercepting, tampering with or falsifying data communicated between the members without recourse to a plurality of encryption keys. In a first step, validity verification data is generated by the terminal. In a second step, the validity verification data and an attestation identity public key certificate are sent from the terminal to a group management device. In a third step, the group management device determines whether the terminal is valid. If the terminal is found valid, then a group name, a member ID and a group-shared key are supplied to the terminal in a fourth step, and a member ID list is further supplied in a fifth step. Thereafter the terminal is allowed to communicate with other members of the same group. The disclosed method can be applied advantageously to video game machines for network-based games.