PURPOSE: To make hardened layer uniform and suppress drop of the strength by performing a pre-processing with such gear dimensions that the angle of twisting becomes greater than the requested value, forming a hardened layer reaching a,certain depth from the surface, and grinding the surface so that the requested angle of twisting is generated.

CONSTITUTION: To fabricate a helical gear, a gear machining. for example hob processing, is made as a pre-processing, wherein the angle of twisting β1 is set greater than the normal angle β as requested. On the surface of each tooth, a hardened layer 9 is formed, for example by a cementation heat treatment, wherein the depth of the layer 9 should be set so that the acute side 11 is deeper than the central part 13 and the obtuse side 15. Then the surface of the tooth is ground by a shaving process till the part of a straight line (a) given by broken line tying Point A reaching the depth Δ=1mm from the surface of the tooth at the end face of the acute side 11 to the Point B on the surface of the tooth at the end face of the obtuse side 15. Thereby the normal angle of twisting β as requested is generated.