The back-up device for an IP-PBX of the present invention is connected to each network. The private branch exchange is connected to an individual network to manage and hold the entire terminal device. The back-up device holds each terminal data of a group of terminal devices in a connected network. As a result, when connection management by the private branch exchange develops a failure, the back-up device secures connection management in the network to which the back-up device is connected. When the network or the private branch exchange itself develops a failure to disable control of an accommodated IP terminal, this arrangement enables the IP terminal to resume operation by temporarily shifting control of the IP terminal to the back-up device side, thereby minimizing stop time.