PURPOSE: To form a hob to a small diameter further to high rigidity, by forming a hob shaft right angular section of an inserted blade of a built-up gear hob to a wide shape toward an axial line while arranging a base of the blade and a perpendicular from the axial line not to pass the center of bottom width of a fitting groove in a hob main unit.

CONSTITUTION: In a built-up gear hob, constituted by closely fitting inserted blades 4 to a hob main unit 3, a hob shaft vertical section of a fitting part of the inserted blade 4 is formed to a wide shape toward an axial direction of the hob main unit 3, while the inserted blade 4 is fitted in such a manner that a bottom surface 4d of the inserted blade and a perpendicular L from a hob shaft are formed not to cross a fitting groove of the hob main unit 3 and not to pass the center of a bottom width direction of the fitting groove. Further a rear side 4c and bottom surface 4d of the inserted blade 4 are formed crossing at a right angle, and grinding and measurement work of the groove in the main unit 3 and the inserted blade 4 can be easily performed.