On the basis of a finding that a magnetic hysteresis of a metal material, among magnetization characteristics changing with secular degradation of the metal material, shows a clear correspondence with the degree of degradation of the metal material, a change in such a magnetization characteristic is measured to estimate the degree of secular degradation of the metal material. In a typical embodiment, a superconducting quantum interference device (33; 340) is used to detect the magnetization characteristic of a measuring object (1; 11). According to the present invention, the degree of embrittlement of a metal material used in an environment of high tempera­tures can be quickly detected in a non-destructive fashion so that the danger of brittle fracture of the metal material can be reliably prevented.