PURPOSE: To prevent the contamination due to hydrocarbon introduced from an airtight maintaining part, and to stably maintain a high airtightness for a long period of time, in a method for maintaining the airtightness of a high purity space, by using a fluororesin compound for the airtight member in the opening/closing part of the space.

CONSTITUTION: A fluororesin compound is used for the airtight member 15 of the open/close door 14 of a storehouse 1 which is equipped with an air cleaner for housing a wafer carrier 10. When the fluororesin compound is used, it is preferable to subject it previously to heat treatment for removing impurities. The heating temperature is to be usually 150°C or more, and to be lower than the softening point or melting point of the material. For example, in the etylene tetrafluoride resin, it is 150 to 300°C. As for the shape when it is used, that which has some elasticity is preferable, and for example, that which has been formed in a sponge, or that which a fluororesin compound has been coated on a sponge material or elastic material that has been formed out of a proper material may be used.