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A depth estimation method for a crack on metal surfaces has been studied and a simple estimation formula and measurement method have been proposed. This formula is based on our previous theoretical analysis that the backscattering from a crack causes periodical s (dips) as the frequency changes, and the first dip occurs when the depth of the crack becomes nearly one-half the incident wavelength. Dependencies of the crack's aperture and the incident angle have also been studied and considered as our depth estimation parameters. Test measurement has been made to check the validity of our estimation procedure.