PROBLEM TO BE SOLVED: To more precisely and nondestructively capture the secular deterioration of a ferromagnetic structural material or ferromagnetic structure.
SOLUTION: In this method, and secular change of the ferromagnetic structural material is nondestructively inspected on the basis of a hysteresis curve represented by the magnetic field strength H and magnetization M measured for the ferromagnetic structural material. The work of dividing the hysteresis curve into a plurality of pieces and determining the gradient of the curve in each division point from the gradient of a straight line connecting the division point concerned to a non-adjacent division point apart by natural number k pieces is performed with respect to plural kinds of natural number k values. For the value of each natural number k, the degree of conversion of the gradient to a relational expression containing magnetic field strength H, magnetic susceptibility χC and the magnetic susceptibility coefficient c showing the degree of secular deterioration of the ferromagnetic structural material is examined to obtain a natural k value having the highest convergence as an optimum value. The magnetic susceptibility coefficient c is determined from the magnetic susceptibility χC in the optimum value, and the degree of secular deterioration of the ferromagnetic structural material is nondestructively inspected on the basis of the magnetic susceptibility coefficient c.