1. A surface inspecting method 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 the steps of: calculating a power spectrum of generation wave generating the flaw of the test object; integrating the power spectrum of the generation wave passing the flaw of the test object and calculating an integration value thereof; collating the calculated integration value by converting the integration value into a 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.