Abstract: In a pressure-controlling apparatus employed in a pre-chamber of an environmental testing chamber for a self-propelled vehicle, in which testing chamber various tests are conducted under a subatmospheric pressure, which testing chamber is provided with the pre-chamber for permitting an opertor's getting in/out of the testing chamber, the improvement resides in that: operational switch means provided in the pre-chamber permit the operator to open/close a first and a second opening/closing valves, which first opening/closing valve is provided in a first air-duct which opens into open air at one end and into said pre-chamber at the other end for recovering an air-pressure of the pre-chamber, which second opening/closing valve is provided in a second air-duct which opens into the testing chamber at one end and into the pre-chamber at the other end for reducing the air-pressure of the pre-chamber.