To provide a method of diagnosing degradation of a stacked layer with embedded a tension member consisting of a magnetic substance which can diagnose interlaminar peeling in the stacked layer with the embedded tension member consisting of the magnetic substance.

The purpose of the method of diagnosing degradation is attained by impressing alternating magnetic field H from exterior of the stacked layer 1 wherein the tension member consisting of the magnetic substance is embedded, exciting the tension member 11 in the stacked layer 1, then generating acoustic sound in the interlaminar peeling part in the stacked layer 1, detecting the sound, and detecting the harmonic of exciting frequency of the alternating magnetic field H and judging the existence of the interlaminar peeling.