A method for supporting a communication service of an idle mode terminal located in a common routing area in a mobile communication system that includes a Node B, an Access Gateway (AGW), and a Home Subscriber Server (HSS), and has a Long Term Evolution (LTE) network and a 3rd Generation (3G) network connected to each other to perform transmission/reception. The AGW receives tracking area information of the LTE network, including routing area information of the 3G network, from the idle mode terminal via the Node B. The AGW sends common routing area indication information and identification information of the terminal to the HSS if the routing area information is coincident with the tracking area information. The HSS stores the common routing area indication information and the identification information of the terminal, and sends a response message according to the indication information.