In the ubiquitous environment, more and more devices are deployed in our daily life, and need to communicate with one another. M2M (Machine-to-Machine) communication is considered to be one of the major issues in future networks. M2M is expected to bring various benefits in wireless communications when it is interconnected with cellular networks. Considering the characteristics of cellular M2M networks, traditional security solutions are not proper to be applied to cellular M2M networks because the M2M network itself is vulnerable to various attacks. We consider security aspects for cellular M2M communications and propose a key management mechanism including the pairwise key and group key establishment. Our proposal could provide reliability and efficiency for the cellular M2M communication network in the secure manner.