A link management device with a special function is connected to the IP network, and the link management device controls a process of establishing a link between an IP converter device and a signal transfer point. The IP converter device transmits a request including a point code to the link management device, and the link management device determines the IP address and the like of the signal transfer point as a connection destination from the received point code. The link management device transmits information for identifying the signal transfer point to the IP converter device, and also transmits information for identifying the IP converter device to the signal transfer point as the connection destination. The port number of the IP converter device is determined by the link management device. The port number of the signal transfer point is determined by the link management device.