Heretofore, GPRS (General Packet Radio Service) standardized by the 3GPP has been known as a technique to perform communications with an external network (PDN: Packet Data Network, for example) by using a cellular phone network. 
GPRS is specified such that an “MM (Mobility Management) procedure” and an “SM (Session Management) procedure” are processed in series. 
For this reason, in the GPRS, when starting communications with an external network (PDN), the user first needs to perform a registration procedure (attach procedure) to a communication network by using the MM procedure, and then to connect to the external network (PDN) by using the SM procedure. 
In this manner, GPRS poses a problem that it takes a long period of time for the user to actually complete the establishment of a connection to an external network (PDN) after initiating a process to start communications with the external network (PDN). 
Meanwhile, as for the “SAE (System Architecture Evolution)” whose standardization is currently under way in the 3GPP, there has been studied a method of shortening the connection delay described above and providing in a pseudo manner a state where the user can transmit and receive an IP packet at any time (“Always On”). 
In the SAE, a scheme is proposed in which the establishment of a connection to an external network (Default IP Service) is completed while the user performs an attach procedure to a communication network, the attach procedure being specified in the “LTE (Long Term Evolution)” (See Non-patent Document 1). 
This scheme will be described with reference to FIG.  1. 
As shown in FIG.  1, when an RRC (Radio Resource Control) connection is established between a mobile station UE and a radio base station eNB in Step S2001, the mobile station UE transmits an attach request to a switching apparatus MME (Mobility Management Entity) and a serving SAE-GW, in order to perform a registration procedure to a communication network (a communication network specified in the LTE), in Step S2002. 
In Step S2003, a process for authenticating the mobile station UE is performed among the mobile station UE, the switching apparatus MME, the serving SAE-GW, and a subscriber information management server HSS (Home Subscriber Server). 
In Step S2004, the switching apparatus MME and the serving SAE-GW transmit “Update Location” to the subscriber information management server HSS, “Update Location” notifying the subscriber information management server HSS of location information of the mobile station UE. 
In Step S2005, the subscriber information management server HSS transmits subscriber information of the mobile station UE to the switching apparatus MME and serving SAE-GW. 
In Step S2006, a U-plane transmission path (route) is established between the mobile station UE and a gateway apparatus PDN serving SAE-GW for an external network (PDN). 
In Step S2007, an IP bearer is set up between the radio base station eNB, the switching apparatus MME and the serving SAE-GW. 
In Step S2008, the switching apparatus MME and the serving SAE-GW transmit an attach response for notifying the mobile station UE of the completion of the registration procedure to the communication network, to the mobile station UE.     Non-patent Document 1: 3GPP TR23.882-180 Section 7.13.2    Non-patent Document 2: 3GPP TS23.401-020