Abstract: The present invention provides a leakage flux flaw detecting method in which a ferromagnetic substance such as a steel sheet or coil is magnetized by a plurality of magnetic fields with different intensity and magnetic flux leaking from the ferromagnetic substance having been magnetized is detected by a magnetic sensor, and output signals of the magnetic sensor after the detection of leakage flux are processed so that a signal caused by a flaw in the ferromagnetic substance is highlighted. Since this leakage flux flaw detecting method makes it possible to detect flaws with high accuracy, a scaled or descaled hot rolled steel sheet or coil in which the accurate flaw information such as location, density, size, and the like is known can be provided by applying this method to the manufacturing of hot rolled steel sheet or coil.