Machine-to-Machine (M2M) networks enabling networked nodes (sensors or actuators) to exchange information have been intensively studied lately. Key components of M2M networks include sensors that can be deployed with a large quantity for monitoring or surveillance. This paper deals with M2M networks hierarchically structured to accommodate efficient data transmission from terminal nodes to a sink node via cluster heads. A novel medium access control (MAC) protocol, more specifically backoff time decision rule, fit for hierarchical M2M networks is proposed and evaluated by simulations. Analytical success probability in channel access by cluster heads is derived and compared with simulation results. It is shown by simulation results that the proposed backoff time decision rule achieves smaller average latency and reduced per-node energy consumption in comparison with well-known MAC protocols, the DMAC and the SMAC, for sensor networks.