A tensile load is applied to the test body to increase with time, and an AE wave displacement in the test body is detected (step S1). From the detected AE wave, waveform data are generated for each time section (step S2). For each section, from the waveform data, spectrum data are generated (step S3), a peak of an intensity in the spectrum data is specified, a data part in which an intensity is at least a value of a set percentage of the peak in the spectrum data is extracted as processing target data (step S4), and from the processing target data, the most frequent value of frequency gravity centers is specified (step S5). The most frequent value for each section and a tensile load applied to the test body in each section are output as strength evaluation data for evaluating a tensile strength of the test body (step S6).