PURPOSE: To shorten the carburizing time and to provide the steel with excellent toughness by incorporating specified amounts of C, Si, Ni, Cr and S into the steel.

CONSTITUTION: The carburizing steel is constituted of, by weight, 0.26 to 0.40% C, 0.5 to 1.5% Si, 1.0 to 2.5% Ni, 0.5 to 1.5% Cr, 0.005 to 0.025% S and the balance Fe. In the steel, C has the properties of shortening the carburizing time, but the effect can not be obtd. for the lower limit or less of its content and the effect of improving toughness by the addition of Ni is impaired for the upper limit or more. Si drastically improves the service life by the synergistic effect with the improvement of toughness by Ni, but the effect is reduced for the lower limit or less and carburizing is impaired for the upper limit or more. Ni shows the above effect, but the effect can not be obtd. for the lower limit or less and, on the contrary, toughness is deteriorated for the upper limit or more. Cr improves hardenability and carburizing properties, but the effect can not be obtd. for the lower limit or less and giant carbide is produced at the time of carburizing to cause the deterioration of the service life for the upper limit or more. S improves workability, but the effect can not be obtd. for the lower limit or less and A-series nonmetallic inclusions are increased and the influence can not be neglected for the upper limit or more.