A processing object such as a steel plate, favorably provide a steaming method can be steamed while suppressing an increase in the processing cost. The present invention relates to a method for producing a water vapor treatment product by contacting with steam treatment object 1 in the sealed container 10, the object to be processed 1 is arranged inside, and a predetermined amount of water 7 in advance in the sealed container 10, which is stored inside the water 7 heated to generate steam, wherein at least provided with a water vapor treatment step of contacting a and the object to be processed 1 the steam. .The