Defect detection device and defect detection method

A defect detection method, comprising:

• a vibration-giving process for giving a test object a vibration of a predetermined frequency;
• a vibration state measurement process for performing a measurement of a vibration state of a measurement area on a surface of the test object by an optical means while the vibration is given to the test object, and for determining, for each position on the surface, a numerical value representing the vibration state based on a result of the measurement;
• a judgment-index-value determination process for determining a judgment index value by performing Fourier transform on an entire area of the measurement area based on the numerical value representing the vibration state at each position, where the judgment index value is a numerical value representing a strength of the vibration of each wavenumber;
• a wavenumber-wavelength determination process for determining, based on a comparison of each of the judgment index value determined for each wavenumber, a wavenumber or wavelength of an elastic wave induced within the entire area of the measurement area in the test object by the vibration given to the test object;
• and
• a determination process for determining whether or not the wavenumber or wavelength determined in the wavenumber-wavelength determination process is within a predetermined range, based on the wavenumber or wavelength as well as a size of a defect expected to be present in the test object,
• wherein, when it is determined in the determination process that the wavenumber or wavelength is not within the predetermined range, the aforementioned processes are once more performed after the predetermined frequency is changed.