In recent years, photovoltaics system (PVS) have been widely used, but the change in characteristics with long-term operation is still not sufficiently clarified yet. We would like to clarify the relationship between the long-term operation and the shunt resistance (Rsh) reduction of cell, which is one of causes of hot spots. In order to clarify these relationships, a method for efficiently evaluation of Rsh reduction is necessary. In this paper, we propose a new method to evaluate Rsh reduction using thermal images. We examined a method to evaluate Rsh reduction efficiently on a string scale by the difference in the heat generation temperature of the shaded cells. As a result, it became clear that the Rsh reduction of cell can be roughly evaluated by the proposed method.