A nondestructive evaluating method for aged deterioration in austenitic stainless steel comprises an information obtaining step for previously obtaining a relationship between an applied stress .sigma. and the first ratio Ms/.chi..sub.H* between a saturation magnetization Ms and a pseudo susceptibility .chi..sub.H* based on reference minor hysteresis loops obtained by applying stresses .sigma., respectively, correspondingly to a result of a tensile test for the same kind of material as an evaluation target. In a measuring step, values of the first ratio Ms/.chi..sub.H* are obtained as measured values from measured minor hysteresis loops obtained by measuring the evaluation target. In an evaluating step, the relationship between the applied stress .sigma. and the first ratio Ms/.chi..sub.H* is compared with the measured values, thereby evaluating aged deterioration of the evaluation target. The minor loops are obtained by stepwise changing a magnetic field amplitude H.sub.a within a range of a magnetic field intensity lower than a saturation magnetization.

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