A complex Fourier transform is performed on measured values of wall thickness at points in a cross section of a pipe in an axial direction, types of thickness deviation are classified, thickness deviation amount is calculated from the absolute value of the complex Fourier component, the position of a thick or thin portion of thickness deviation is calculated from the phase of the complex Fourier component, and manufacturing conditions of the pipe are adjusted based on these variables. The relationship rexp(j.theta.) between the thickness deviation amount r and phase .theta. of a first-order thickness deviation obtained for cross sections in the axial direction is subjected to a complex Fourier transform as a function of pipe longitudinal direction, the thickness deviation is further classified by the frequency of twist of the thickness deviation, and an action is taken to prevent the thickness deviation according to the classified thickness deviation.