In 2017, specifications for highway bridges in Japan was revised from allowable stress design to limit state design with partial factors. Since large earthquakes frequently occur in Japan, it is necessary to develop a reasonable and reliable load-carrying capacity evaluation method, which can take the partial plasticization of members into account in order to make a bridge work effectively. In this study, experimental data was obtained from bending tests for a steel girder with reinforced concrete slab. In particular, the influences of plastic neutral axis position, depth to thickness ratio of a web, the arrangement of reinforcing bars in the slab and the effect of composite action between the girder and the slab were investigated. As a result, concrete crushing of the slab occurred after reaching full plastic moment not only in the case designed by the current specifications but in the case that depth-to-thickness ratio of the web was increased compared with current specifications. In addition, the arrangement of reinforcing bars made possible to improve toughness after the maximum load. These results become useful information on the establishment of limit state design in Japan as well as maintenance of existing bridges.