Whenever we employ the longitudinal cut method of wave analysis in the towing tank, we are worried by the effect of the local disturbance near the model ship. It was found that the effect could be evaluated from the value of the Fourier transform of the wave profile in the range of the wave number lower than g/V ² (= K ₀). In this paper we have been able to make clear usefulness of the wave analysis as a technique in the experimental tank.