PURPOSE: To greatly increase the strength of the tooth by making the pitch surface non-cylindrical, and realizing the line-contact with a counterpart gear over a large part of the tooth width during the engagement.

CONSTITUTION: The pitch surface of a line-contact crossed helical gear is formed curved like a drum, and manufactured by a gear hobbing machine and a hob cutter. The desired curved surface is machined not by straightly moving the hob cutter parallel to the axis of rotation of a work, but by being driven for machining in a non-straight manner. When the machine-driving curve of this hob cutter is appropriately selected, a screw gear which realizes the line-contact over a large part of the tooth width during the engagement can be obtained. The strength can be greatly increased by this crossed helical gear, and such screw gears can be mass-produced at a low cost by a regular hob cutter.