There have been many researches focusing on IP-bases wireless sensor networks for environmental monitoring, building management, and measurement of devices" status information. In that networks, topology management function which controls network entities such as status of sensor nodes is required. In the previous works, sensor gateway is usually used for inter-connection between sensors and external Internet devices. In this paper, we design and implement a sensor gateway architecture for efficient topology management system that sensor node supports COAP protocol. Using this system, it can be observed sensor nodes to add or change by using COAP protocol. We demonstrate the efficiency for topology management of our architecture by comparing the energy consumption for the number of wireless signaling with current designs.