Abstract: A server transmits an encryption key or encryption key information for specifying the encryption key to a mobile terminal. The mobile terminal acquires play money at the game machine, generates encrypted information by encrypting an ID token with the received encryption key or an encryption key corresponding to the received encryption key information, and transmits the ID token and the encrypted information to the server through the game machine. The server decodes the encrypted information, transacts a payment based on a user ID identifying a user of the mobile terminal and the amount of play money when the ID token received by the server matches the ID token acquired by the decoding, and authorizes the game machine to let the user play up to the amount of play money. Therefore, the game can be started by exchanging electronic data between the mobile terminal and the game machine with high security.