PURPOSE: To lengthen the life of a cutter as well as to enable the best tooth contact to be obtained by cutting gear by means of a conventional method while a hob cutter having a pitch circle diameter smaller than the pitch circle diameter of a worm gear is moved along the circular arc of a specified radius around the center axis of the worm.

CONSTITUTION: By the use of a hob cutter 3 with a pitch circle diameter (d) smaller than the pitch circle diameter D of a worm, a worm gear is cut by means of a conventional method while the center axis O of the cutter 3 is being moved along a circular arc having a turning radius of (D-d)/2 around the center axis of the worm to be meshed with a worm gear to be formed. Namely, a work piece 1 is rotated around the cutter axis O at revolutions meeting requirements, when the center axis O is concurrently moved from a point O to a point O6 along the locus 6 by a center movement, the pitch circle 31 of a cutter 5 is moved to a pitch circle 36. An envelope 5 formed by the pitch circle of the cutter becomes identical to an envelope formed when a cutter is used, which is exclusively used for the worm gear, thereby letting the work piece 1 be formed with the worm gear having smooth surfaces.