In this study, the fatigue damage in type 304 stainless steel was estimated using an inherent magnetic sensor via plane-bending fatigue tests and the electromagnetic impedance method. The sensor was a magnetic composite material incorporating a ferromagnetic martensite phase generated in type 304 stainless steel by a surface finish process during the production stage. The output properties of this sensor as a function of the number of cycles were evaluated under various conditions. It was demonstrated that this sensor could detect fatigue damage starting from the zeroth cycle. The sensor output repeatability was evaluated, and the variation in the output between the inherent magnetic sensors was approximately 10% regardless of the sensor type and total strain amplitude. By using the two proposed estimation methods, the specific fatigue level and the number of cycles could be estimated with errors of 3–27%. These results indicated that the inherent magnetic sensor was suitable for use for fatigue damage estimation.