1. That is, when the reception quality is higher than the threshold level, the terminal status manager 23 determines that the mobile terminal 3 is located in the cell of the home network HPLMN (alternatively, the mobile terminal 3 has moved from the cell of the visited network VPLMN to the cell of the home network HPLMN).

Note that, when the reception quality is lower than the threshold level, the terminal status manager 23 determines that the mobile terminal 3 is not located in the cell of the home network HPLMN. In this case, the VPLMN base station 2 waits for the next measurement report.

When the reception quality becomes higher than the threshold level, the terminal status manager 23 transmits an update request to the VPLMN core network 5. This update request indicates that the mobile terminal 3 has moved into the cell of the home network HPLMN. In the VPLMN core network 5, the mobility manager 51 creates an EPLMN update instruction based on the update request. Here, this update request indicates that the mobile terminal 3 has moved to the cell of the home network HPLMN. Therefore, the EPLMN update instruction instructs that the value of the EPLMN of the mobile terminal 3 be updated to a value representing the home network HPLMN. That is, the EPLMN update instruction instructs that the PLMN number representing the home network HPLMN be recorded as the network with the highest priority in the PLMN list 34 of the mobile terminal 3. The mobility manager 51 then transmits an EPLMN update instruction to the mobile terminal 3.

Note that the procedure to create and transmit the EPLMN update instruction to the mobile terminal 3 is performed by the connection controller 51b illustrated in FIG. 2. That is, when the communication quality between the mobile terminal 3 and the home network HPLMN becomes higher than a specified threshold level, the connection controller 51b issues an instruction to the mobile terminal 3 that the mobile terminal 3 should preferentially connect to the home network HPLMN.

Upon receiving the EPLMN update instruction via the VPLMN base station 2, the mobile terminal 3 updates the PLMN list 34. In this example, as illustrated in FIG. 6B, the PLMN number representing the home network HPLMN is recorded as the network with the highest priority in the PLMN list 34.

Furthermore, in the VPLMN core network 5, the mobility manager 51 transmits an update response corresponding to the update request to the VPLMN base station 2. Thus, the terminal status manager 23 creates a release request according to the update response and sends the release request to the RRC processor 21.

Upon receiving the release request, the RRC processor 21 releases resources for communication between the mobile terminal 3 and the VPLMN base station 2. The RRC processor 21 then transmits a release notification to the mobile terminal 3. The release notification indicates that a resource for communication between the mobile terminal 3 and the VPLMN base station 2 is released due to “redirect”. Therefore, upon receiving the release notification, the mobile terminal 3 releases the resources for connecting to the visited network VPLMN. The RRC processor 21 also transmits a release notification to the VPLMN core network 5. Thus, the mobility manager 51 records, in the subscriber information storage 52, that the mobile terminal 3 has moved away from the visited network VPLMN.

The mobile terminal 3 starts a procedure to connect to a new network. At this time, the mobile terminal 3 attempts connections, among the networks registered in the PLMN list 34, in order starting with the network with the highest priority. In this example, the PLMN list 34 is updated to the state illustrated in FIG. 6B by the above-described EPLMN update instruction. Therefore, in this case, the mobile terminal 3 attempts to connect to the home network HPLMN. Specifically, the mobile terminal 3 sends an Attach request to the HPLMN base station 1. The Attach request is then forwarded to the mobility manager 41 of the HPLMN core network 4. As a result, the mobile terminal 3 is capable of connecting to the home network HPLMN.

As described above, in the procedure according to the embodiment of the present disclosure, when the mobile terminal 3 is connected to the visited network, the mobile terminal 3 measures the quality of the signal received from the base station of the home network, based on an instruction (the measurement report request in FIG. 3) transmitted from the visited network. When the reception quality is higher than the threshold level, the core network of the visited network (in FIG. 2, the mobility manager 51) transmits, to the mobile terminal 3, information (in FIG. 4, an EPLMN update instruction) designating a network to which the mobile terminal 3 should preferentially connect. The mobile terminal 3 then connects to a network designated from the core network of the visited network. That is, the mobile communication system is capable of controlling switching of the connection from the visited network to the home network based on the radio status of the mobile terminal 3.

In this case, when the mobile terminal 3 connected to the visited network moves into the cell of the home network, the procedure illustrated in FIG. 4 is performed immediately, and the mobile terminal 3 connects to the home network. Therefore, the time required for switching from the cell of the visited network to the cell of the home network is short.

In addition, the EPLMN (that is, the PLMN list 34) of the mobile terminal 3 is updated by the EPLMN update instruction transmitted from the mobility manager 51 of the visited network. Therefore, when the mobile terminal 3 has moved from the cell of the visited network to the cell of the home network, the mobile terminal 3 is capable of reliably re-connecting to the home network. In the example illustrated in FIG. 7, when the mobile terminal 3 moves from the point P2 to the point P3, the mobile terminal 3 is located in both the HPLMN cell 1a belonging to the home network and a cell 6a belonging to another network. However, a home network HPLMN is registered in the PLMN list 34 of the mobile terminal 3. Therefore, even in such a case, the mobile terminal 3 is capable of reliably re-connecting to the home network.

Note that, in the foregoing example, when the communication quality becomes higher than the threshold level, cell switching is immediately performed, but the present disclosure is not limited to this procedure. For example, when the communication quality becomes higher than the threshold level, the VPLMN base station 2 may check whether data communication is continuous between the mobile terminal 3 and the VPLMN base station 2, as illustrated in FIG. 4.

In this case, when data communication is continuous between the mobile terminal 3