The authors have developed such sensors which can be applied to steel structures for health monitoring. The sensor is manufactured by placing copper foils (electrodes) on polymer film and splaying the electrically conductive material on the surface. The electrically conductive material is composed of resin and carbon particles. Carbon particles form a continuous link each other. The strain due to the deformation of the steel members influences the electrical resistance. We can evaluate the strain of the steel members by measuring changing rate of the electric resistance, and then estimate the damages of the steel members. A strain monitoring system with this sensor enables low cost, because it does not need special equipment except for an electrical resistance meter. The authors carried out experimental tests on steel structure specimens with the film sensor attached. The specimens were beam-to-column connections of steel structures buildings subjected to alternative cyclic deformation correspond to input by severe earthquakes. Based on the tests, we investigated the performance and applicability of the sensor for the members of steel structures. As the test results, it turned out that the sensor is able to be used for strain detecting of steel structures.