A subzero treatment method of heat-treated steel is provided to extend life of a heat-treated steel product such as a mold and improve productivity by using a refrigerant maintaining a liquid state at room temperature, thereby universalizing the subzero treatment method through simple handling of the refrigerant. A subzero treatment method of heat-treated steel comprises the processes of: installing a container(40) filled with silicone oil which is in a liquid state at room temperature and has a solidification point of -80 to -150 deg.C within a subzero treatment apparatus(10); and performing subzero treatment of the heat-treated steel by dipping a heat-treated steel into the silicone oil and gradually dropping temperature of the heat-treated steel such that the heat-treated steel and silicone oil are thermally balanced.
