A method and apparatus for providing roaming service in a wireless communication system. A roaming gateway for performing communication between an Internet protocol multimedia subsystem (IMS) network and a circuit switched (CS) network receives an authentication request for a roaming mobile station (MS), which roams from an IMS network to a CS network, from a mobile service center (MSC) of the CS network, acquires authentication information for the roaming MS and service information for services, with which the roaming MS has been provided from the IMS network, at the authentication request, checks if the roaming MS is entitled to use the services according to the service information by using the authentication information and the service information, and sends an authentication success message to the MSC when the roaming MS is entitled to use the services.