A method of performing a security mode control procedure by a user equipment (UE), and an apparatus therefor are disclosed. The method includes performing, over a 3rd generation partnership project (3GPP) access, a first authentication procedure and a first key agreement procedure with an access and mobility management function (AMF), wherein a key set identifier (ngKSI) is changed during the first authentication procedure and the first key agreement procedure, receiving, from the AMF over the 3GPP access, a first security mode command message including the ngKSI, and receiving, from the AMF over a non-3GPP access, a second security mode command message including the ngKSI, wherein the UE is registered to the AMF and a same public land mobile network (PLMN) over both the 3GPP access and the non-3GPP access.