A packet data unit session establishment method and network entity performing the same is disclosed. User equipment (UE) may establish registration with the core network through a first access network when the UE is registered in a core network through a second access network belonging to the network and a first PDU session is established through the second access network. The UE may transmit a non-access stratum (NAS) message including a PDU session identifier (ID) that is the same as an ID of the first PDU session to an access and mobility management function (AMF).