This invention provides a simple means for detecting a renal disorder, a diagnostic marker for a renal disorder that enables prognostic prediction of a renal disorder and evaluation of the degree of nephropathy at the phase of stage-II diabetic nephropathy by measuring the megalin level in urine associated with a renal disorder used for the detection means, and use of such marker. Firewalls are provided at each end of each private tunnel which recognize IP packets addressed to devices at the other end of the tunnel and encapsulate these packets in other IP packets which have a header which includes as the destination address, the IP address of the untrusted side of the firewall at the other end of the tunnel. This reduces the forward resistance of the electronic switching device in the switched-on state.