An apparatus for controlling an internal pressure of a hermetically sealed chamber at a prescribed pressure level serves to relax pressure changes in the hermetically sealed chamber. The chamber is connected to an inlet passage for supplying air from an external environment through an aseptic filter, an outlet passage for discharging a gas through an aseptic filter, and an inlet valve and an outlet valve for selectively opening and closing the inlet passage and the outlet passage, respectively. The apparatus also includes a controller for controlling the inlet valve and the outlet valve to keep the internal pressure of the chamber at a prescribed positive pressure level. An elastic membrane of silicone rubber which is expanded or retracted depending on a change in the internal pressure of the chamber to relax the pressure change is connected to the outlet passage between the aseptic filter and the outlet valve.
