Abstract: A decompression apparatus comprises an exhaust line connected to a processing unit, an exhaust device connected to the exhaust line, whereby the processing unit is exhausted through the exhaust line, and an ambience in the processing unit can be set under a negative pressure, a main valve in the exhaust line for opening and closing the exhaust line, a bypass line having one end portion, connected to that portion of the exhaust line which is situated nearer to the processing unit than the main valve, and the other end portion connected to that portion of the exhaust line which is situated on the exhaust side of the main valve, the exhaust flow quantity of the bypass line per unit time being lower than that of the exhaust line, a sub-valve in the bypass line for opening and closing the bypass line, and a control unit adapted to close the main valve and the sub-valve as the pressure in the processing unit is increased from the negative pressure to the normal pressure and to open only the sub-valve to keep the ex