Carburizing process is one of the heat treatment which consumes energy in large quantities. Authors started the program to develop new carburizing steels which would be able to shorten the process time by 10 to 20% comparing with conventional steels. To obtain the data for alloy design, the effect of alloying elements was investigated throughly on the case depth of carburized steels. Then authors learned that C and Mo are most effective to gain the case depth and designed new carburizing steels, ES1 and ES2, where contents of 2 elements were optimized. ES1 has machinability, core - hardenability and tensile/fatigue strength equivalent to SAE 5120 (Cr steel). And ES2 are to substitute for JIS SCM 420 (Cr - Mo steel). ES1 passed the application experiments as steering components of a passenger car.