PURPOSE: To realize a voice mail box function which has high secrecy by adding a dummy incoming subaddress, which is generated on a receiving side at the time of a 1st incoming call, on both the calling side an receiving side thereafter and performing operation processing.

CONSTITUTION: A mail device 2 which receives a call setting message from a network knows an incoming call for a data communication. Then a control part 212 retrieves an incoming subaddress information element group in the call setting message. When no incoming subaddress is detected, the control part 212 generates a dummy subaddress corresponding to a message, sent from the calling side thereafter, by a dummy subaddress random generator 212i and sends an answer message having necessary contents to the calling side. On the receiving side, on the other hand, a message which is sent back under the control of the control part 212 begins to be detected and is recorded and stored in an area corresponding to the dummy incoming address. Consequently, a voice mail box system which is superior in the secrecy of a mail function is realized.