(A) first data for defect amount estimation for the guided wave of a first frequency is obtained, the data indicating a relationship among amplitude of the reflected wave, a defect cross-sectional area and a defect width. (B) second data for defect amount estimation for the guided wave of a second frequency is obtained, the data indicating a relationship among amplitude of the reflected wave, a defect cross-sectional area and a defect width. (C) a guided wave of the first frequency is generated, and amplitude of a reflected wave is detected as first amplitude. (D) a guided wave of the second frequency is generated, and amplitude of a reflected wave is detected as second amplitude. (E) on a basis of the first and second data and the first and second amplitude, a defect cross-sectional area and a defect width of the defect part are estimated.