We proposed a seismic retrofitting method for existing R/C building with low strength concrete.In the proposed method,a reinforcing frame composed of an H-section peripheral frame and pin-ended circular hollow section bracings is placed on the outer surface of an R/C frame.Experimental studies were carried out to evaluate the reinforcing effect.The specimen is about 1/3-scale model for two story-two bay substructure frame of an R/C building.A tube-in-tube brace,which is restrained not to buckle by the outer tube,is applied as a reinforcing brace.In the experiment,the horizontal shear loads were cyclically applied to the specimen under a constant vertical load.Through the experimental results and analytical results,an effectiveness of the proposed method is discussed.The horizontal load carrying capacity of the specimen is agreed with predicted the value approximately for the reinforcement of an R/C frame.