A fatigue detecting sensor that enables easy and precise estimation of fatigue damage of machines and structures was developed, based on the crack growth characteristics of metal. The sensor is composed of a metal foil with a single edge notch and a base metal foil. The two metal foils are bonded at the both far ends. The sensor is attached with an adhesive to the surface of structural member which is subjected to cyclic loads. The fatigue damage of the member is detected as the fatigue crack initiation from the notch tip of the sensor. The fatigue crack growth rate is independent of the crack length. The metal foil of the sensor is given pre-tension, so the sensor has stable crack growth characteristics independent of the average strain. The crack growth length is measured in a certain period after the installation. The length is converted to the fatigue damage of the member during the sampling period of sensor installation. Since no measuring instrument and signal wiring is necessary, the diagnosis procedure becomes easy and the cost for structural health monitoring could be reduced. With such small size of the sensor as conventional strain gages, the pin-pointed application of the sensor to the hot spot of structural stress concentration, where is important for the fatigue strength estimation of welded structures, is possible. Therefore, precise fatigue remaining life assessment could be carried out. Sensors with several strain ranges were developed, so application to vast strain range is possible. Even such a small strain range as 1/10000 is able to be detected. The sensors have been applied to various products like steel bridges, rolling stocks, ships, etc.