A method identifies a cause of change in magnetic permeability of an object under inspection and evaluates the state of surface treatment(s) of the object under inspection with high accuracy. An embodiment of the method includes preparing a non-destructive inspection apparatus; placing the object under inspection; generating eddy current in the object under inspection; continuously changing the penetration depth of an AC magnetic field in the object under inspection; calculating the value of impedance at each penetration depth in the object under inspection; and evaluating the state of the surface treatment(s) by: calculating the ratio between the value of impedance at each penetration depth in the object under inspection and the value of impedance at a corresponding penetration depth in steel which has not been subjected to a surface treatment; and identifying a cause of a change in magnetic permeability of the object under inspection based upon the calculated ratio.