A method of heat-treating a steel member includes a reduced-pressure carburization step a1 of carburizing a steel member in a carburizing gas under a reduced pressure, a reduced-pressure cooling step a2 of cooling the steel member after the reduced-pressure carburization step a1 in a cooling gas whose pressure is reduced to lower than atmospheric pressure, and a hardening step a3 of heating a desired part of the cooled steel member using high-density energy. The reduced-pressure cooling step is preferably performed while the cooling gas is stirred under the condition that the pressure of the cooling gas is reduced to lower than atmospheric pressure. The reduced-pressure cooling step is preferably performed at least from a time before which a structural transformation of the steel member due to cooling is started to the time when all the structural transformation is completed. The pressure of the cooling gas during the reduced-pressure cooling step is preferably in the range of 0.1 to 0.65 bar.