Abstract: A high vacuum apparatus which comprises:a vacuum vessel,a mechanical booster connected to the vacuum vessel,a rotary pump connected to the mechanical booster,a gas inlet provided with a connecting pipe which connects the mechanical booster with the rotary pump, to introduce a gas into the suction side of the rotary pump so as to maintain the degree of vacuum in the suction side of the rotary pump lower than the maximum degree of vacuum attainable of the rotary pump, so that substantially no contamination of the atmosphere in the vacuum vessel by the reverse diffusing oil vapor occurs.