PROBLEM TO BE SOLVED: To apply bending processing to a plastic pipe with high productivity. SOLUTION: A plastic pipe is curved by applying cold bending processing to a desired section of the plastic pipe. Subsequently, the plastic pipe 1 is attached to the support stand 3 of a restriction device 2 by a restricting jig 4 to be restricted to a curved state and introduced into a furnace in this state and heated to temp. from the thermal deformation temp. of the plastic pipe to the m.p. thereof to be held for a proper time to be heat-treated and the relaxation of stress generated by bending processing is advanced and permanent bending deformation strain is allowed to remain to form a bending processing part.