To solve the problem that it takes a time for a conventional monitor system to find a fault of a monitored-side device of a system in continuous operation by a method for making a self-diagnosis only when the monitored-side device is powered on.

At a request to establish a connection of a socket from one client device 2a, a server device 1 establishes the connection of the socket and then a self-diagnosis processing part 13 takes a self-diagnosis; if the result of the self-diagnosis shows abnormality, the connection of the socket is disconnected and if a connection establishment request for a socket is received from another client device 2b while the result of the self-diagnosis indicates normalcy and the connection of the socket with the client device 2a is maintained, a self-diagnosis is not newly made and only the connection establishment processing for the socket is performed.