A distortion computation method, the method comprising: detecting a profile of a steel sheet rolled by a rolling mill; computing from the detected profile second distortion indicating distortion appearing in an undulation on a surface of the steel sheet that constitutes deviation from a desired value of the rolled steel sheet; characterized in that the method comprises: determining third distortion from correlation between a threshold value of the third distortion indicating distortion corresponding to internal stress of the rolled steel sheet and a wavelength of a rolling-direction component of a profile determined from the detected profile, and the wavelength of the rolling-direction component of the detected profile, wherein the correlation is computed by buckling analysis from a boundary condition determined from the detected profile, thickness of the rolled steel sheet, width of the rolled steel sheet, tensile strength of the rolled steel sheet, and distribution pattern of the third distortion; and adding the second distortion to the third distortion so as to compute first distortion indicating difference between distortion corresponding to stress applied to the steel sheet by the rolling mill and desired distortion of the steel sheet by the rolling. detecting a profile of a steel sheet rolled by a rolling mill; computing from the detected profile second distortion indicating distortion appearing in an undulation on a surface of the steel sheet that constitutes deviation from a desired value of the rolled steel sheet; characterized in that the method comprises: determining third distortion from correlation between a threshold value of the third distortion indicating distortion corresponding to internal stress of the rolled steel sheet and a wavelength of a rolling-direction component of a profile determined from the detected profile, and the wavelength of the rolling-direction component of the detected profile, wherein the correlation is computed by buckling analysis from a boundary condition determined from the detected profile, thickness of the rolled steel sheet, width of the rolled steel sheet, tensile strength of the rolled steel sheet, and distribution pattern of the third distortion; and adding the second distortion to the third distortion so as to compute first distortion indicating difference between distortion corresponding to stress applied to the steel sheet by the rolling mill and desired distortion of the steel sheet by the rolling. determining third distortion from correlation between a threshold value of the third distortion indicating distortion corresponding to internal stress of the rolled steel sheet and a wavelength of a rolling-direction component of a profile determined from the detected profile, and the wavelength of the rolling-direction component of the detected profile, wherein the correlation is computed by buckling analysis from a boundary condition determined from the detected profile, thickness of the rolled steel sheet, width of the rolled steel sheet, tensile strength of the rolled steel sheet, and distribution pattern of the third distortion; and adding the second distortion to the third distortion so as to compute first distortion indicating difference between distortion corresponding to stress applied to the steel sheet by the rolling mill and desired distortion of the steel sheet by the rolling.