When a fault is detected by a surveillance agent mounted in a user device, fault information is reported to a maintenance center device by an electronic mail through an internet. The user device is provided with a connect confirmation unit to prepare and transmit connect confirmation mails of a plurality of patterns different in mail content, and the maintenance center device is provided with a diagnosis unit, which diagnose a mail transmission environment of the user device from the reception result of one or a plurality of connect confirmation mails received within a fixed period of time and transmits a diagnosis result to the user device by the electronic mail.