A gas exhaust unit evacuates the inside of a vacuum transfer chamber at a constant exhaust rate. An gas exhaust valve is kept normally open, and a purge gas (N2 gas) is supplied from a purge gas supply source into the vacuum transfer chamber via a mass flow controller (MFC) and an opening/closing valve. A main control unit controls a pressure in the vacuum transfer chamber to be within a specified range through a flow rate set value for the MFC while monitoring a pressure in the vacuum transfer chamber via a vacuum gauge. The main control unit determines occurrence of abnormality when the pressure exceeds a specified upper limit and then takes such actions as changing a flow rate set value for the MFC, giving an alarm and stopping the operation of a vacuum processing apparatus.