Seismic retrofit has been achieved for the buildings constructed before 1981, and its effects on the improvement to avoid collapse due to an earthquake are reported in the past investigation. Public office, school buildings and gymnasium were retrofitted as priority to secure disaster prevention base in the city. However, it is pointed out that the damaged commercial facility and factory lead to the terrible economic loss in the 2011 Tohoku great earthquake. Although seismic retrofit is needed for factories with lack of seismic performance, the continuous operation causes many temporal and spatial restriction. Especially, retrofit with welding makes the construction impractical because it should pay attention to prevention of fire. Fundamental experimental study on a non-fire strengthening method for bolted angle brace connections were addressed in this paper. In other words, monotonic loading tests were carried out to investigate the effects of the proposed retrofit method. The proposed method in this paper is that the additional member is attached to the existing angle brace by new high strength bolts, the joint bolts. First of all, the ultimate strength for the two expected failure modes was established to select the test parameters. The expected failure mode I is fracture occurred not only at the existing bolt hole but also at the joint hole, and including the diagonal failure line from the existing bolt hole to the joint bolt hole. The failure mode II is fracture occurred at the joint bolt hole only because the joint bolt is located far enough from the existing bolt hole. It indicates that the position of the joint bolt from the existing bolt is an important parameter to determine the failure mode of the retrofit method.
 The test results showed that the additional member increases the ultimate strength of the existing bolted connection. The effects depend on the position of the joint bolt and the ultimate strength increases according to the distance from the existing bolt to the joint bolt, the joint distance x. And although the ductile crack was observed around the 1st existing bolt in specimen with short joint distance, the position of the crack was changed to around the 1st joint bolt in specimen with wide joint distance. It indicates that the failure mode is changed to the failure mode II. The increment of ultimate strength was nearly constant in the failure mode II, and it means that the retrofitted connection is able to reach the maximum strength. Based on the test results with a scatter of material strength on cross section, it is found out that the joint distance has to be 1.5 times or more of the width of the angle cross section in order to change the expected failure mode to mode II, and to maximize the ultimate strength of the bolted connection.