To reduce the shrink quantity of a pipe end generated at a time of cutting without performing long-time annealing by forming a molded product so that the tensile stress shown by the distribution of the internal stress in the axial direction of the molded product toward the wall thickness direction thereof shows the max. value on an outer surface and the distribution of a degree of crystallization in the wall thickness direction is min. on the outer surface and max. on an inner surface.

The tensile stress shown by the distribution of axial internal stress σ (z1) calculated by formula in a wall thickness direction shows the max. value on an outer surface and the distribution of the degree of crystallization of a molded product in the wall thickness direction is min. on the outer surface and max. on an inner surface. Herein, E is the longitudinal modulus, ν is a Poisson's ratio, z0 is the distance from the center 0 of the wall body of a tubular member to the surface of the wall body and quantity shown by ± on inner and outer surfaces, z1 is the distance between the surface obtained by removing the wall body of the tubular member from the surface over arbitrary depth in a laminar state and the center 0 of the wall body of the tubular ember and (Z1) is a curvature after the deformation of the wall body calculated by (Z1)=8f/L2 when the warpage quantity of the wall body developed by removing the wall body of the tubular member from the surface by (Z0-z1) in a laminar state is set to (f) and the length of the wall body is set to L.