There is disclosed a technique which can detect a packet routing loop (in particular, a nesting loop formed by MRs (mobile routers). According to this technique, an MR 114 connected as a subordinate to an MR 110 transmits a BU (Binding Update) message, to which added is an ARO (Access Router Option) including an address of a router (MR 110) connected at present, to an HA (Home Agent) 140 to which it pertains. Moreover, an MR 112 is connected as a subordinate to the MR 114 and, when connected to the MR 112, the MR 110 transmits a BU message, to which added is an ARO including an address of the MR 112, to the HA 140. At this time, the MR 112 also transmits a BU message, to which added is an ARO including an address of the MR 114, to the HA 140. Thus, the HA 140 can detect a routing loop formed by connections 120, 122 and 124.