Recently, a number of fatigue cracks have been detected in orthotropic steel decks in Japan. An effective inspection method for detecting and monitoring crack is strongly needed. In this study, Field Signature method (FSM), which is one of the Non-destructive inspection methods and has been already applied to detect corrosion or a section reduction of the plumbing in a power station and various plants, was focused. FSM system was applied to a couple of real steel bridges which were under different conditions during this study. Finally, an effectiveness of the system for monitoring fatigue cracks on steel bridges has been verified.