A method, an apparatus, and a system for solving and managing security problems, which may occur during a handover of a User Equipment (UE) between PLMNs in a mobile communication network, by using a Non-Access Stratum (NAS) protocol are provided. By the method, a UE can perform a security mode command and an authentication with a network. Further, the method can prevent interruption of communication due to authentication or security during a handover of a UE between Public Land Mobile Networks (PLMNs).