Corrosion is one of important factor for securing the safety of steel bridges. In general cases, the corrosive environment of the steel bridge is evaluated as a site environment. However, even in one bridge, the corrosive environment greatly varies from part to part. This research aims to clarify the difference of corrosion environment for each part of plate-girder-bridge which has three main girders. At this bridge, anti-freezing agent is sprayed in winter. On site measurements were performed on five points on each girder, which are both sides of web, both sides of upper/lower part of bottom flange. These measurements points include two points where water leakage is scattered. Temperature, humidity, amount of airborne salt and amount of adhering salt have been measured. In order to comprehensively assess corrosive condition, exposure test was also performed. Observations were carried out for one year. From measurement results, it became clear that temperature and humidity were not uniform at all observation points. These were differed at the inside and outside of girder and upper part and lower part of web. Amount of airborne salt to each girder is strongly influenced by anti-freezing agent. On the upper surface of the lower flange of each girder, there are places where corrosion markedly progresses due to deposits and water leakage.