A method for processing a non-access-stratum, NAS, request message performed at a mobility management function, MMF, node, the method comprising: receiving the NAS request message from a terminal; checking which access network the NAS request message from the terminal has been received via; checking, when the NAS request message from the terminal is confirmed to have been received via a second access network, whether there are a first mobility management, MM, context and a first security context which have been generated during a previous registration process through a first access network, based on information included in the NAS request message; obtaining a second security context, generated during an authentication procedure for the second access network, from an authentication control plane, CP, node; generating a second MM context based on at least one of the first MM context and the first security context, and the second security context obtained through the second access network; transmitting a response message to the NAS request message to the terminal through a packet data gateway, PDG; wherein the terminal creates a plurality of sessions through the first and second access networks, wherein the MMF node recognizes the first and second security contexts based on an identifier, ID, of the authentication CP node, wherein the terminal is identified by the information contained in the NAS request message, which is a Global Unique Temporary Identity, GUTI, wherein the NAS request message further includes information on the authentication CP, wherein, if the first MM context is updated based on the second security context, the second security context present in the authentication CP node is released, and wherein, the first MM context and the second MM context are created and managed in access units. receiving the NAS request message from a terminal; checking which access network the NAS request message from the terminal has been received via; checking, when the NAS request message from the terminal is confirmed to have been received via a second access network, whether there are a first mobility management, MM, context and a first security context which have been generated during a previous registration process through a first access network, based on information included in the NAS request message; obtaining a second security context, generated during an authentication procedure for the second access network, from an authentication control plane, CP, node; generating a second MM context based on at least one of the first MM context and the first security context, and the second security context obtained through the second access network; transmitting a response message to the NAS request message to the terminal through a packet data gateway, PDG; wherein the terminal creates a plurality of sessions through the first and second access networks, wherein the MMF node recognizes the first and second security contexts based on an identifier, ID, of the authentication CP node, wherein the terminal is identified by the information contained in the NAS request message, which is a Global Unique Temporary Identity, GUTI, wherein the NAS request message further includes information on the authentication CP, wherein, if the first MM context is updated based on the second security context, the second security context present in the authentication CP node is released, and wherein, the first MM context and the second MM context are created and managed in access units.