PURPOSE: To execute a timing of a read message when a message not read is deleted at a high speed by limiting a management object of the message not read back from a message to be registered at last to a prescribed number so as to minimize the data quantity as required.

CONSTITUTION: Various information of a message such as message registration date, registration party and its content is generated to reference the information of a message management section 3 and a conference information management section 2. Number of unread messages (e.g. 1024) as a management object and a message number registered at present are compared. When the latter message number is more than 1024, each bit of delete message identification information is shifted to the left and '0' is added to a final bit. Thus, the final 1024 messages are always managed in the deleted message identification information. Then the data managed by the message management section 3 and the conference information management section 2 is revised by the managed message. Thus, the managed data of unread message is decreased.