DESCRIPTION Title of Invention GROUP SECURITY IN MACHINE-TYPE COMMUNICATION Technical Field This invention provides a security solution for group based Machine-Type Communication (MTC) which is disclosed in non patent literature 1 and 2 . The invention intends to reduce the signaling between MTC devices and network and establish efficient secure communication between group based MTC devices and the network. Background Art MTC communication has been drawing attention in both the technical/academic and industry. According to the current 3GPP (3rd Generation Partnership Project) specifications, the MTC devices in the same area and/or have the same MTC feature attributed and/or belong to the same MTC user can group together and communicate to network as a unit. However, from security point of view it is not yet described that how the MTC devices that belong to a group establish secure communication with network, including the authentication to devices and key allocation. Neither that how a MTC device establishes secure communication with network when it newly joins a group. A method to establish such a secure communication between communication devices is disclosed in patent literature 1. In patent literature 1, when a distributing server, which distributes data, receives a distributing request from another server (requesting server), the distributing server sends a security request to the requesting server. The requesting server performs setting for its security and sends information on the security setting. The distributing server confirms the security setting of the requesting server, and then sends requested data to the requesting server if there is no problem on the security setting received. Furthermore, related arts are disclosed in patent literature 2, 3 and 4 as follows. In patent literature 2, a method by using hush function to generate a temporary identifier (ID) for a new wireless communication device which does not conflict with IDs of the other wireless communication devices, is disclosed. A communication method between communication devices is disclosed in patent literature 3 . One-to-one or one-to-multiple communication is performed between devices which are set as the same group. In patent literature 4 , it is disclosed that an integrating entity integrates messages received from a plurality of entity, and send the integrated message to its destination . Patent literature 5 discloses a M2M (Machine to Machine, Mobile to Machine, Machine to Mobile) module which is connected to a network. [Citation List] [Patent Literature] [Patent Literature 1] Japanese Patent Application Laid-Open No. 2008-257340 [Patent Literature 2] International Patent Publication No. WO2007/07281 4 [Patent Literature 3] Japanese Patent Application Laid-Open No. 2006- 081 84 [Patent Literature 4] Japanese Patent Application Laid-Open No. 2007- 0891 56 [Patent Literature 5] Japanese Patent Application Laid-Open No. 2008- 5431 37 [Non Patent Literature] [Non Patent Literature 1] TS 22.368 "Service requirements for Machine-Type Communications (MTC); Stage 1" (Release 10). [Non Patent Literature 2] TS 23.888 "System Improvements for Machine-Type Communications" (Release 10). [Non Patent Literature 3] TS33.401 "3GPP System Architecture Evolution (SAE); Security architecture" (Release 9). Summary of Invention [Technical Problem] However, patent literature 1 discloses secure communication between single communication devices, in other words, one-to-one communication. Therefore, if the secure communication method disclosed in patent literature 1 is applied to the system supposed in non patent literature 1, which includes a plurality of the MTC devices, traffic in a network would increase in proportion to the number of MTC devices. This is because that the communication is carried between each single MTC device and the network. [Solution to Problem] There are two main practical situations considered. One is the group has been created and the group ID (grID) is shared by the MTC devices that belong to the group and the network before any communication started. For the devices, the grID is embedded in its USIM (Universal Subscriber Identity Module) card. This will be presented in Invention . The other situation is for a MTC device to join an existing group. The network does not have knowledge about this MTC device in advance. However the MTC devices who satisfy the network's requirement for a group can request to join the group. In the opposite of invention I, the network and the MTC device can not reach an agreement in advanced. The solution will be proposed in Invention II. It is optional for gateway to have an UICC (Universal Integrated Circuit Card) . When a MTC device functions as a gateway, it will have an UICC. An exemplary object of the invention is to provide a communication apparatus, a communication system, a communication method and a storage medium for storing a communication program which can solve the problem described above. A communication apparatus to an exemplary aspect of the invention, which is connected to a network and a plurality of communication terminals, includes: a group information sending means for sending group information which is received from the network; an access control means for receiving a reply from the communication terminal which responded to the group information and for sending the reply to the network; and a temporary identifier and group key sending means for sending a temporary identifier and a group key to the communication terminal which responded to the group information, when the communication apparatus received the temporary identifier and the group key from the network. A communication system to an exemplary aspect of the invention includes: a plurality of communication terminals; a network; and a communication apparatus which relays communication between the plurality of communication terminals and the network; wherein the communication apparatus receives group information from the network, sends the group information to the plurality of communication terminals, and sends a temporary identifier and a group key to the communication terminal replied to the group information. A communication method to an exemplary aspect of the invention, which is performed between a network and a plurality of communication terminals, the method includes: receiving group information from the network; sending the group information to the plurality of communication terminals; receiving a reply to the group information from the communication terminal; and sending a temporary identifier and a group key to the communication terminal replied to the group information. A storage medium for storing a communication program to an aspect of the invention includes: a group information sending process for sending group information which is received from the network; an access control process for receiving a reply from the communication terminal which responded to the group information and for sending the reply to the network; and a temporary identifier and group key sending process for sending a temporary identifier and a group key to the communication terminal which responded to the group information, when the temporary identifier and the group key are received from the network. [Advantageous Effects of Invention] According to the present invention, traffic between MTC devices and the network can be decreased and- a secure communication will be established between the group based MTC devices and the network. Brief Description of Drawings [Fig. 1] is a block diagram for the Invention I. [Fig. ] is a block diagram for the Invention I I . [Fig. 3] is a message sequence chart between MTC device, gateway and core network in the Invention I. [Fig. 4] is a message sequence chart between MTC device, gateway and core network in the Invention I I . [Fig. 5] is a block diagram showing a configuration of the communication apparatus in the first exemplary embodiment. [Fig. 6] is a flowchart showing an operation in the first exemplary embodiment. [Fig. 7] is a block diagram showing a configuration of the communication system in the second exemplary embodiment. [Fig. 8] is a block diagram showing a configuration of the gateway in the second exemplary embodiment. [Fig. 9] is a block diagram showing a configuration of the MTC device in the second exemplary embodiment. [Fig. 10] is a block diagram showing a configuration of the core network in the second exemplary embodiment. [Fig. 11] is a flowchart showing an operation in the second exemplary embodiment. [Fig. 12] is a flowchart showing an operation in the third exemplary embodiment. Description of Embodiments [Invention I] The object of the invention is achieved by using a gateway (GW) for security management to group optimized MTC devices. Main role of the gateway is to establish security communication between the MTC devices and the core network, distribute group key (grKey) and unicast temporary IDs to the MTC devices, and optionally perform access control and generate temporary IDs. A few assumptions are made for this invention as below. 1. The gateway and the core network (CN) have established secure communication. 2. The group is created beforehand on network decision. 3. The unique group D is known by all of the MTC devices within the group and retrieved by the gateway from the network before any communication is started. 4 . The authentication between the gateway with UICC and the network and, between the MTC devices and the network follows 3GPP standard AKA (Authentication and Key Agreement) . 5 . Each gateway can manage more than one group. The invention consists of the steps below. 1. The gateway broadcasts the grIDs and sets a timer. A MTC device will respond to the gateway with a matched grID it holds. 2 . The gateway sends a concatenated Attach Request message to the network for the MTC devices responded before the timer is expired. 3 . Access control against MTC list for the MTC devices responded the broadcast is performed by ( 1 ) the gateway only, (2) the network only, (3) or both the gateway and the network. 4. AKA procedure for the MTC devices is performed in the way that all the messages from the MTC devices are collected by the gateway and sent to the network in a concatenated message. In the same way, the message sent from the network is a concatenated message and the gateway will distribute to each MTC device. 5. After a successful AKA procedure, a Security Mode Command (SMC) procedure is performed as that of 3GPP standard [3] . From which, the integrity and confidential keys are generated and activated for communication between the MTC devices and the network. 6. The gateway receives grKey from the network after the secure communication is established between them, and before the gateway distributes it to the MTC devices. The gateway can optionally generate the grKey itself. 7. The network creates a unique temporary ID (tempID) for each MTC devices, by which it can exclusively recognize and communicate with a MTC device. It sends the templDs to the gateway, and the gateway unicasts the tempID to each MTC device. The gateway can optionally create tempID for a MTC device. In this case, it will send the templDs to the network. Fig. 3 is a message sequence chart between the MTC device, the gateway and the core network in the Invention . At step 10, the gateway and the core network perform mutual authentication and establish secure channel. At step 2 , the network sends the grID, the gwlD, the grKey, the group feature and the MTC list to the gateway. The grKey can be optionally generated by the gateway. At step 14, the gateway broadcasts the grID with features of a group, and starts a timer to wait the MTC device's response. At step 6, the MTC device which has stored the grID and the feature matched with those in the broadcast shall respond the broadcast. At step 18, the gateway will perform access control for the MTC device which responded its broadcast, by comparing the received grID against the MTC list. When the timer is expired, any response from the MTC devices will be discarded. The access control here is optional if the network will perform access control. At step 20, the gateway sends a concatenated Attach Request message including all the Attach Request messages from the MTC devices. At step 22, the network will perform access control against the MTC list. This procedure is optional if the gateway performs access control and the network trusts it. At step 24, the network performs authentication procedure with the MTC devices, followed by a Security Mode Command (SMC) procedure in step 26. At step 28, the network (or the gateway optionally) generates a unique temporary ID for each MTC device. If the network generated the templDs, it will send them to the gateway in the Attach Accept message. If the gateway generated templDs, it will inform them to the network in step 32. At step 30, the gateway unicasts the templDs and distributes the grKey to the MTC devices. [Invention I I ] The object of the invention is achieved by a strategy that the network broadcasts feature requirements of a group, since the network does not have any previous knowledge about the MTC devices which are to be in the group. The MTC devices which meet those feature requirements will (request to join a group) be authenticated individually by the network. Assumption 1, 4 , 5 of Invention I applies here. And a few other assumptions are made for this invention as below. . The network and the MTC devices do not have any knowledge about each other beforehand. 2. Mutual authentication between the network and the MTC devices, and identity allocation follow 3GPP standard procedure. The invention consists of the steps below. 1. The network broadcasts features of a group. 2 . The MTC devices which match the feature may respond, e.g. by sending an Attach request to join the group. 3 . The network performs authentication and access control for the MTC devices. 4 . The network generates and sends the tempID to the MTC devices. 5 . The network informs the gateway which MTC device will join the group by sending the MTC device identity to the gateway. 6 . The generation of tempID can optionally be done by the gateway, if so, the network will send the IMSIs (International Mobile Subscriber Identity) of the MTC devices to the gateway. The gateway will send the network the templDs after the generation. 7. The gateway distributes the grKey to the MTC devices. Fig. 4 is a message sequence chart between the MTC device, the gateway and the core network in the Invention l|. At step 10 , the gateway and the core network perform mutual authentication and establish secure channel. At step 12, the network broadcasts the group features and starts a timer to wait the MTC device to respond. At step 14, the MTC device which matches the features can respond by sending a request of joining the group. At step 6 , the network will perform access control against the MTC list. When the timer is expired, any response from the MTC devices will be discarded. At step 18 , the network will perform authentication with the MTC device which responded its broadcast, followed with a SMC procedure. At step 20, the network will generate a tempID which is unique for each MTC device. At step 22, the network will send the tempID to the MTC device in the Attach Accept message. At step 24, the network will indicate the gateway the succeeded authenticated MTC devices' tempID. At step 28, the gateway distributes the grKey to the MTC devices, and can optionally unicast the templDs to the MTC devices which can be generated in step 26. If the gateway generates the templDs, it will send them to the network. According to the embodiments described above, sending concatenated messages between the gateway to the network decreases signaling and provides efficiency especially when the group size grows large. Using a gateway for the security management of a group, it prevents further attacks to a network. It provides flexibility for a group to have new members. Access control tempID generation locally performed by the gateway can also reduce the network's load. Hereinafter, the exemplary embodiments of the present invention are described in detail with reference to accompanying drawings.