To provide a magnetic flaw detecting method and magnetic flaw detecting device for highly accurately detecting flaws on a material to be inspected extending in various directions, using a rotating magnetic field.

The magnetic flaw detecting device 100 includes a magnetization means 1 for acting a rotating magnetic field to the object to be inspected, a detection means 2 for detecting a flaw-detection signal, and a signal processing means 3 for providing signal processing to a flaw-detection signal. The magnetization means energizes an alternating current superposing a first current and a second current with a lower frequency than that of the first current as an excitation current. The signal processing means includes a first synchronous detection means 31 for synchronously detecting the flaw-detection signal by regarding the first current as a reference current, a second synchronous detection means 32 for synchronously detecting the output signal of the first synchronous detection means by regarding the second current as a reference signal, and extracting a flaw-candidate signal, and a flaw-image displaying means 34 for displaying a flaw-detection image, in which each pixel has concentration responding to intensity of the flaw-candidate signal in each part of the material to be inspected and a phase of the flaw detection signal in each part is recognized.