PURPOSE: To allow the tooth surface of a hardened external gear to be ground with high accuracy in a short time and at low cost by electrodepositing hard abrasive grain, containing CBN or the like, to the internal tooth surface of base metal formed of an alluminum alloy into approximately annular shape with internal teeth formed.

CONSTITUTION: The base metal 1 of an internal gear type grinding wheel is to be formed of A390 material (Japan light metal) with silicon content being 12% or more. At the time of machining the tooth part 2 of the base metal 1, gear cutting is directly applied to solid material by grinding using a diamond electrodeposition-formed grinding wheel 10 having a forming tooth form part 9 at the tip, and then hard abrasive grain (CBN grain) 3 is electrodeposited to the internal gear face by electrodeposition machining. The items of an aluminum alloy internal gear electrodepositing tool at this time are module: about 2.75, pressure angle: about 14.5°, tooth number: about 84, angle of torsion: about 27° right, and face width: about 30mm, for instance.