Machine-to-Machine (M2M) communication has attracted considerable attention as a new communication paradigm that increases wireless network traffic. Since existing mobile cellular networks need to persistently maintain a tunnel for each mobile terminal in their core system, the number of concurrent tunnels increases dramatically when a large number of M2M devices are handled in mobile cellular networks large popularity. The small Average Revenue Per User (ARPU) of M2M terminals also has a strong impact on network cost. In this paper, we propose a tunnel sharing method to reduce the cost of accommodating M2M terminals to mobile cellular networks. In the proposed method, multiple terminals are aggregated as a group and share a single tunnel in the mobile core network. We also propose introduction of temporary tunnels to avoid call blocking caused by competition for a single tunnel among simultaneous calls. We evaluate the performance of the proposed method by mathematical analysis to examine its basic characteristics. We also estimate the monetary cost to realize the proposed method and reveal that it can reduce at 80% compared with traditional mobile core networks, while keeping the call blocking rate less than 0.1%.