Angle brace is one of the important components for resisting earthquake in low-rise steel structure building. However， some fractures at the pre-1981 bolted connection have been observed even in the recent earthquake. Seismic retrofitting method for the existing brace connection should be developed with not only effective but also easy applications. In this paper， bolted seismic retrofitting method is proposed and experiments were carried out to investigate its ultimate strength. From the test results， evaluation method for ultimate strength was established with the critical section change due to the position of the reinforcement.