A distortion correction method includes: a reference surface forming process for fabricating a reference surface as a circumferential surface in an outer circumference of a raw pipe having both axial ends to which fitting members are welded; a connecting member installation process for installing a reference surface of the raw pipe in a roller contact member that rotatably supports the raw pipe; a measurement process for measuring a tilt of the fitting member rotating along with the raw pipe by virtue of the roller contact member with respect to an axial direction of the raw pipe; and a distortion correction process for performing TIG welding for a weld portion between the raw pipe and the fitting member on the basis of a result of the measurement in the measurement process to correct the tilt of the fitting member with respect to the axial direction of the raw pipe.