The newly emerging 5G mobile communication system requires an effective design of core network to cope with the mobile data traffic explosions. In this paper, we propose a new architecture for 5G mobile core network, RB-Core, to efficiently deal with such the data traffic explosion problem. The proposed RB-Core network is designed by adopting a distributed architecture and by using the Routing Bridge as a key component. By numerical analysis, it is shown that the proposed architecture can provide some advantages over the existing 4G core network architecture in terms of tunnel overhead and transmission delay.