We Claim:
1. A mobile communication system in which a first base station and a second
base station communicate with a user equipment in parallel,
wherein the first base station and the second base station transmitor receive, to or from each other, a notification regarding lease of the second base station from a state wherethe first base station and the second base station communicate with the user equipment in parallel,
the first base station transmits, to the user equipment, a notification regarding lease of the second base station from a state where the first base station and the second base station communicate with the user equipment in parallel, and
the use equipment transmits, to the first base station, a notification regarding lease of the second base station from a state where the first base station and the second base station communicate with the user equipment in parallel.
2. The mobile communication system as claimed in claim 1,
wherein the first base station transmits, to the second base station, a notification regarding lease of the second base station from a state where the first base station and thesecond base station communicate with the user equipment in parallel.
3. The mobile communication system as claimed in claim 1,
wherein the second base station transmits, to the first base station, a notification regarding lease of the second base station from a state where the first base station and the second base station communicate with the user equipment in parallel, and
the first base station transmits, to the second base station, a notification regarding lease of the second base station from a state where the first base station and the

second base station communicate with the user equipment in parallel.
4. A mobile communication system in which a first base station and a second
base station communicate with a radio network controller in parallel,
wherein the first base station transmits, to the radio network controller, a notification regarding lease of the second base station from a state where the first base station and the second base station communicate with theradio network controllerin parallel, and
theradio network controller transmits, to the first base station, a notification regarding lease of the second base station from a state where the first base station and the second base station communicate with theradio network controllerin parallel.
5. The mobile communication system as claimed in claim 4,
wherein the radio network controller includes:
a mobility management device that communicates with the first base station in a control plane; and
a gateway station that communicates with the second base station in a user plane,
the mobility management device transmits, to the gateway station, a notification regarding lease of the second base station from a state where the first base station and the second base station communicate with theradio network controllerin parallel, and
the gateway station transmits, to the mobility management device, a notification regarding lease of the second base station from a state where the first base station and the second base station communicate with theradio network controllerin parallel.

6. The mobile communication system as claimed in claim 4, wherein the second base station transmits, to the first base station, data received from the radio network controller.
7. A base station that communicates with a user equipment in parallel with another base station,
wherein the base station transmits or receives, to or from the other base station, a notification regarding lease of the other base station from a state where the base station communicates with the user equipment in parallel with theother base station,
the base station transmits, to the user equipment, a notification regarding lease of the other base station from a state where the base station communicates with the user equipment in parallel with theother base station, and
the base station receives, fromthe user equipment, a notification regarding lease of the other base station from a state where the base station communicates with the user equipment in parallel with theother base station.
8. A base station that communicates with a user equipment in parallel with
another base station,
wherein the base station transmits or receives, to or from the other base station, a notification regarding lease of the base station itself from a state where the base station communicates with the user equipment in parallel with theother base station.
9. A user equipment that communicates with a first base station and a second
base station in parallel,
wherein the user equipment receives, from the first base station, a notification

regarding lease of the second base station from a state where the user equipment communicates with the first base station and the second base station in parallel, and
the user equipment transmits, to the first base station, a notification regarding lease of the second base station from a state where the user equipment communicates with the first base station and the second base station in parallel.