The present invention provides a method of heating and pasteurizing in use of gaseous water and an apparatus therefor, and the present invention relates to a method of heating and pasteurizing a material to be treated, comprising the steps of: continuously injecting hot water heated to at least 100.degree. C. and/or steam into a heating chamber comprising a semi-sealed space and heated to at least the same temperature as the hot water and/or steam, to form water microdroplets and moist hot steam; replacing the air within the heating chamber with the water microdroplets and moist hot steam to effect filling with a gas component that has a composition of at least 95% humidity and an oxygen concentration not greater than 1% and that is maintained in the temperature range of 90 to 180.degree. C.; and carrying out a heating and pasteurizing treatment by effecting, with the water microdroplets and the moist hot steam, continuous amplitude heating at a temperature difference of at least 10.degree. C. in the aforementioned temperature range on a material to be heated and pasteurized, and also relates to an apparatus for this method.