PURPOSE: To let a sender confirm the contents of communication which could not be transmitted by storing a communication message, by converting its transmitting destination into the transmitting side terminal of the communication message when stagnation longer than a prescribed period is caused after the communication message is stored.
CONSTITUTION: At the time of the registration of an MB, a date at this point of time is preserved previously in a mail box management table in a main storage part 8, and it is checked whether the measured time of a clock part 10 exceeds a term of validity set beforehand or not. Besides, of the above- mentioned measured time exceeds the term validity, the communication message in a main box storage part 9 having exceeded the term of validity is read out. Then, a converting part 11 is started, and the transmitting destination is converted by converting a 'receiversname' frame into a 'sender's name' frame. Next, the communication message converted by the converting part 11 is stored again on the storage part 9. By above-mentioned processing, the transmitting side terminal is informed of the contents of the communication message stagnated for a long period in the MB.