When carrying workpieces from a loading area in which the workpieces are handled into a heat treatment furnace to make the workpieces subjected to a heat treatment process using a predetermined process gas, the loading area is evacuated and controlled at a predetermined low negative pressure. An exhaust for evacuating the loading area is connected to the loading area, and a controller controls the exhaust so that the loading area is maintained at the predetermined low negative pressure. A specific gas and particles contained in a gas discharged from the loading area are removed by filters.