Steel corrosion is one of the causes of steel bridge deterioration. In order to corrode the steel, water and oxygen are required. As measuring the water on the steel surface, the wet time is calculated by temperature and humidity or using an ACM sensor etc. However, those methods cannot measure the water inside of the rust layer directly. This research aims to evaluate the water retention of the rust layer, the electrical resistance was measured. First, the electrical resistance under the wet-dry cycle was measured for specimens which have four grades of rust thickness, under 200μm, 200 to 400μm, 400 to 800μm and more than 800 μm. As a result, the measured resistance was smaller under a wet environment and the trend was the same in all thickness specimens. Second, the electrical resistance with supplying different water quantities was measured for three grades of rust thickness specimens. For the same water quantity, the larger the rust thickness, the smaller the electrical resistance was. Thus, there is a possibility to evaluate the water retention of the rust layer based on the water quantity in the rust by measuring electrical resistance.