18% Ni maraging steel has begun to be used for mould and core for die casting of aluminum and zinc alloys, because it has superior qualities such as good toughness and little change in dimension after heat treatment. But its cost is the bottleneck. 18% Ni maraging steel contains much expensive alloying elements such as Ni, Co, Mo and it is needed to be melted in vacuum because it contains active alloying elements Al and Ti, consequently 18% Ni maraging steel is too expensive for general use. The purpose of this study is to develop a new maraging steel which has similar properties to, and is less expensive than 18% Ni maraging steel. As a result, a new steel whose composition is 0.1C-8Ni-8Cr-5Mo-5Co was developed. This new steel was confirmed to satisfy the purpose of this study mentioned above, so that it is expected to be used widely for aluminum and zince die casting tools.