Method of gas carburizing

In a first process of a method of gas carburizing, a steel treatment object in a carburizing atmosphere containing carburizing gas is heated to an initial set temperature which is not higher than a peritectic point at which δ iron and a liquid phase transform into γ iron and not less than a eutectic point at which the liquid phase transforms into γ iron and cementite, such that the surface carbon concentration thereof does not exceed a solid solubility limit. In a second process following the first process, the carburizing temperature is gradually decreased from the initial set temperature such that the surface carbon concentration of the treatment object increases without exceeding the solid solubility limit, and such that the carburized depth of the treatment object increases.