Automobile transmissions gears have been required to have high strength, high accuracy, and low cost. Further, in the production line of gears, stable quality, high productivity, energy saving, low environmental load, and improved safety have been demanded. In recent years, the environment has changed greatly due to CASE, and the demand for gears will become more sophisticated and complex in the future. Therefore, we will discuss the evolution and the problems of conventional surface-modified gears, and explain mild carburized gears that simultaneously achieved some difficult problems.