A common phone number which is used only among a specific group consisting of members registered in a network is ensured to enable transmitting/receiving a message file at an arbitrary point of time. A management terminal in a network sends identification names of prescribed communication terminals corresponding to an issued common phone number to a phone number management center so as to register the communication terminals in correspondence with common phone number. A message file sent from an arbitrary one of the communication terminals to the common phone number is sent to all the other registered communication terminals or left in a message center, and the arrival of the message file is notified to all the other registered communication terminals. A message sent from an arbitrary one of the communication terminals to the common phone number can also be left on a bulletin board, allowing other registered communication terminals to read the message file left in the message center or the message left on the bulletin board at any time.