The corrosion environments of individual parts of members in a steel structure vary significantly. Therefore, efficient maintenance requires accurately evaluating these environments and predicting corrosion damage over time. This research focused on comprehensively evaluating the corrosive environment of each part of members in the structures based on the mean corrosion depth of uncoated steel plates mounted on the members. Atmospheric exposure tests were conducted in four exposure fields in which the environments varied widely as a function of rainfall and airborne sea salt. Based on the test results, a practical method of estimating the mean corrosion depth using the thickness of the corrosion-product layer on the mounted plates that does not require removal of the corrosion product is proposed. Furthermore, a method for estimating the time-dependence of corrosion depth for parts of the members after deterioration of the paint coating is proposed.