PURPOSE: To miniaturize and as well as cheaply manufacture the captioned device by a method wherein a turbine molecular pump is applied for the air exhaustion of the vacuum vessel and its rotational frequency is controlled continuously.

CONSTITUTION: The vacuum vessel 21 is so constituted that the air within the vessel 21 is exhausted by means of the turbine molecular pump 23 and an oil rotary pump 24, both of which are connected in series to the vessel 21, through a valve opening variable type main valve 25, through which the gas is induced. The pressure within the vessel 21 is detected with a pressure gauge 26 and the detected pressure signal is compared with the set pressure signal in a control circuit 27, which sends a control signal to bring the difference between the above-mentioned two pressure signals to zero to a power source device 28. Because the pump 23 is driven by means of the power source device 28, rotational frequency of the pump 23 is controlled continuously.