In order to improve security upon distributing a group key, there is provided a gateway (20) to a core network for a group of MTC devices (10_1-10_n) communicating with the core network. The gateway (20) protects confidentiality and integrity of a group key, and distributes the protected group key to each of the MTC devices (10_1-10_n). The protection is performed by using: a key (Kgr) that is preliminarily shared between the gateway (20) and each of the MTC devices (10_1-10_n), and that is used for the gateway (20) to authenticate each of the MTC devices (10_1-10_n) as a member of the group; or a key (K_iwf) that is shared between an MTC-IWF (50) and each of the MTC devices (10_1-10_n), and that is used to derive temporary keys for securely conducting individual communication between the MTC-IWF (50) and each of the MTC devices (10_1-10_n).