PURPOSE: To provide the titled mold that is composed of divided parts in a number of (n) for molding a lead screw out of a plastic, by providing the inner circumferential surface of a cylindrical cavity with special ridges and a circumferential reference ridge.

CONSTITUTION: A groove 7 of a lead screw has such a shape that the bottom is narrowed by (L) and the side walls are converged, and the inner surface of a cylindrical cavity has ridges 11, 12 for molding the groove 7. The (L) should satisfy the expression: L≥P{cos-1(r/R)sin(θ/2)-cos-1sin(θ/2)}/360 wherein P stands for the pitch of the groove; R stands for the radius of the shaft; (r) stands for the radius of the bottom of the groove; and θ stands for the angle by which the mold is divided. A circumferential reference ridge is provided for positioning such that the helical ridge of the mold continues smoothly.