When pressure in a chamber is brought to atmospheric pressure and the chamber is filled with an inert gas atmosphere, the atmosphere in the chamber is sucked into an oxygen concentration analyzer through a sampling line such that oxygen concentration in the chamber is measured by the oxygen concentration analyzer. When the pressure in the chamber is reduced to less than atmospheric pressure, nitrogen gas is supplied to the oxygen concentration analyzer through an inert gas supply line simultaneously with suspending the measurement of oxygen concentration in the chamber. Even when the measurement of oxygen concentration in the chamber is suspended, reverse flow to the oxygen concentration analyzer from a gas exhaust pipe can be prevented, and the oxygen concentration analyzer can be prevented from being exposed to exhaust from the chamber. The configuration results in maintaining measurement accuracy of the oxygen concentration analyzer in a low oxygen concentration range.