A method of registering a user equipment, LTE, at a second network via a first network, the method being carried out by an access and mobility management function, AMF, of the first network in a wireless communication system, the method comprising: receiving (operation 1, Fig. 3, Fig. 4), from the UE, a registration request message including identification, ID, information of the LTE in the first network, second network ID information, and ID information of the LTE in the second network; selecting (operation 5, Fig. 3, Fig. 4) an authentication server function, AUSF, of the first network which is configured to perform a first authentication procedure of service subscription authentication for the LTE in the first network; performing (operations 6 and 7, Fig. 3, Fig. 4) the first authentication procedure with the AUSF of the first network and a unified data management, UDM, of the first network; determining (operation 9, Fig. 3, Fig. 4), based on a result of the first authentication procedure, whether to perform a second authentication procedure of service subscription authentication for the LTE in the second network; performing (operation 10, Fig. 3, Fig. 4), based on a result of the determining, the second authentication procedure with an AUSF of the second network and a UDM of the second network; and transmitting (operation 13, Fig. 3, Fig. 4), to the UE, a registration acceptance message comprising information of at least one of the result of the first authentication procedure and a result of the second authentication procedure, wherein the determining of whether to perform the second authentication procedure comprises determining to perform the second authentication procedure based on determining that the first authentication procedure is successful. receiving (operation 1, Fig. 3, Fig. 4), from the UE, a registration request message including identification, ID, information of the LTE in the first network, second network ID information, and ID information of the LTE in the second network; selecting (operation 5, Fig. 3, Fig. 4) an authentication server function, AUSF, of the first network which is configured to perform a first authentication procedure of service subscription authentication for the LTE in the first network; performing (operations 6 and 7, Fig. 3, Fig. 4) the first authentication procedure with the AUSF of the first network and a unified data management, UDM, of the first network; determining (operation 9, Fig. 3, Fig. 4), based on a result of the first authentication procedure, whether to perform a second authentication procedure of service subscription authentication for the LTE in the second network; performing (operation 10, Fig. 3, Fig. 4), based on a result of the determining, the second authentication procedure with an AUSF of the second network and a UDM of the second network; and transmitting (operation 13, Fig. 3, Fig. 4), to the UE, a registration acceptance message comprising information of at least one of the result of the first authentication procedure and a result of the second authentication procedure, wherein the determining of whether to perform the second authentication procedure comprises determining to perform the second authentication procedure based on determining that the first authentication procedure is successful.