PURPOSE: To carry out the inexpensive and stable heat treatment of a steel product accompanied by no decarburization independently of the thickness of an oxide film to be formed by controlling the amount of water or steam to be introduced into a gaseous nitrogen atmosphere in a furnace so as to regulate the partial pressure of oxygen to a prescribed value.

CONSTITUTION: Water or steam is introduced into a gaseous nitrogen atmosphere in a furnace so as to make the steam content about 40W80%, the concn. of oxygen in the furnace is measured, and the amount of water or steam to be introduced is controlled so as to keep a prescribed partial pressure of oxygen. The formation of a decarburized layer is prevented, and an iron oxide layer is formed.