Existing gymnasiums can not satisfy the load-carrying capacity in the seismic capacity evaluation because of non-load-carrying-capacity joint of beam-to-column connections. This reason is caused by using fillet-weld in stead of full-penetration-weld. In these connections rib-strengthening technique is generally employed as an earthquake resistant construction method. In this paper, experimental and FEM analytical study on the beam-to-column connection strengthened by the rib-plate was carried out. The following conclusions were obtained from the results: 1) Resistant mechanism of the rib-plate was clear, 2) position of the plastic hinge of the beam moved from the beam-end to the edge of the rib-plate.