In Japan, a lot of steel railway bridges were constructed during the period of rapid economic growth in the 1960s. Now, since over 50 years have passed, the aging of these structures has become conspicuous and a major social problem. In particular, for steel plate girders with open deck system, deterioration and damage due to corrosion are becoming pronounced. Nevertheless, the remaining strength evaluation and its reinforcement methods of corroded structures are not always sufficient. Therefore the purpose of this study is to propose an analytically-based evaluation method and a reliable measure to reinforce in the maintenance field of steel railway bridges.