1. A surface inspecting method, implemented on a surface inspection device, for inspecting a flaw of a test object using a surface wave and estimating a depth of the flaw of the test object from an attenuation ratio of a frequency of a generation wave, the surface inspecting method comprising: classifying the test object into a sound portion and a flawed portion at a time of estimating the depth of the flaw of the test object; calculating a power spectrum of a generation light passing through the classified sound portion; calculating weighting for each frequency component to be evaluated in the sound portion; calculating a power spectrum of generation wave passing the classified flawed portion; calculating weighting for each frequency component to be evaluated in the flawed portion; calculating an amount of attenuation for each frequency component to be evaluated based on the weighting value for each frequency component in the classified sound portion and the weighting value for each frequency component in the classified flawed portion; collating the calculated amount of attenuation with a database for converting the amount of attenuation into the flaw depth for calibration created beforehand and calculating the flaw depth of the test object; and displaying the calculated flaw depth of the test object.