A method for measuring an S/N ratio in an eddy current testing on an internal surface of a pipe or tube comprising the steps of: separating an eddy current signal obtained by executing an eddy current testing on the internal surface of a pipe or tube with no artificial flaws formed therein in an axis direction into an X-axis component and a Y-axis component that are different from each other by 90° in the phases thereof to obtain signal waveform data of the X-axis component and signal waveform data of the Y-axis component; excluding predetermined low-frequency components respectively from the signal waveform data of the X-axis component and the signal waveform data of the Y-axis component thus obtained, by using an operation unit; calculating a noise voltage value V1 defined by the following Equation based upon a voltage value X(i) of the signal waveform data of the X-axis component with the low-frequency component excluded therefrom and a voltage value Y(i) of the signal waveform data of the Y-axis component with the low-frequency component excluded therefrom, by using the operation unit; preliminary storing a voltage value D of an eddy current signal corresponding to a predetermined artificial flaw obtained by executing an eddy current testing on an internal surface of a pipe or tube with the artificial flaw formed therein in the operation unit, and calculating an S/N ratio defined by the following Equation based upon the voltage value D and the noise voltage value V1, by using the operation unit: [Equation 1] V 1 = π / n ⋅ ∑ i = 1 n X i 2 + Y i 2 1 / 2 where n represents the number of samplings of the signal waveform data, [Equation 2] S / N ratio = D / V 1 separating an eddy current signal obtained by executing an eddy current testing on the internal surface of a pipe or tube with no artificial flaws formed therein in an axis direction into an X-axis component and a Y-axis component that are different from each other by 90° in the phases thereof to obtain signal waveform data of the X-axis component and signal waveform data of the Y-axis component; excluding predetermined low-frequency components respectively from the signal waveform data of the X-axis component and the signal waveform data of the Y-axis component thus obtained, by using an operation unit; calculating a noise voltage value V1 defined by the following Equation based upon a voltage value X(i) of the signal waveform data of the X-axis component with the low-frequency component excluded therefrom and a voltage value Y(i) of the signal waveform data of the Y-axis component with the low-frequency component excluded therefrom, by using the operation unit; preliminary storing a voltage value D of an eddy current signal corresponding to a predetermined artificial flaw obtained by executing an eddy current testing on an internal surface of a pipe or tube with the artificial flaw formed therein in the operation unit, and calculating an S/N ratio defined by the following Equation based upon the voltage value D and the noise voltage value V1, by using the operation unit: [Equation 1] V 1 = π / n ⋅ ∑ i = 1 n X i 2 + Y i 2 1 / 2 where n represents the number of samplings of the signal waveform data, [Equation 2] S / N ratio = D / V 1