Abstract: In a method and apparatus for removing gas from the interior of a hollow-fiber permeability apparatus in which a permeating region of a housing contains a bundle of hollow fibers, and materials can selectively permeate through the membranes formed by the permeable walls of the hollow fibers, between first and second fluids flowing through the housing inside and outside the fibers respectively, at least one of first and second materials to produce carbonic acid gas by mutual chemical reaction is contained in a flow path for the first fluid and/or another flow path for the second fluid, and the first and second materials react with each other in at least one part of the apparatus to produce carbonic acid gas, whereby the gas previously existing in the interior of the housing is substituted with the produced carbonic acid gas.