In order to reduce the noise of transmission gears meshing with each other, high efficiency grinding method of heat-treated gear has been developed. It is adopted for mass-production of gears. These gear grinding methods are called gear honing method and hard finish method. For these methods a diamond dressing gear is used in order to form or dress the tooth profile of grinding wheel. It is a very simple grinding method that a profile of a dressing gear is transcribed on a grinding wheel, and that the tooth flank of gear is finished up with this grinding wheel. The machining time is shorter than of other conventional gear grinding methods so that it is more suitable for mass-production of gears.