Abstract: A structural steel seismic-resistant beam-to-column connection is provided with a pair of rib plates installed symmetrically above and below the connection. The rib plates are perpendicularly to an upper and a lower beam flange and disposed at positions corresponding to the center of the beam web. The rib has a variable cross section at its front end and a relatively lower lengthened section at its rear end. Through the use of the rib plate, the stress concentration at those fan type weld access holes of the beam web as well as the junction of the full penetration weld between the beam flange and column flange can be alleviated, and the plastic hinge can be limited to the beam at a specified portion and can develop excellent plastic rotation capacity in the case of a severe earthquake.