This invention relates to resin material for gas separation base containing a cardo polyimide structure in which the hydrogen atoms in the side-chain benzyl and/or allyl position are halogenated at a rate of modification by halogen of 0.1% or more or resin material for gas separation base containing polymer in which the hydrogen atoms in the side-chain benzyl and/or allyl position are halogenated at a rate of modification by halogen of 34% or more and, additionally, relates to polymer which serves as raw material for the aforementioned resin material for gas separation base containing a cardo polymer structure and a process for producing said polymer; said polymer not only excels in such properties as solvent solubility, ease of conversion to film by a wet process, thermal stability, and chemical stability but also performs well in gas permeability and the process of this invention makes it possible to produce gas separation base, particularly gas separation membrane, whose gas permeability and gas selectivity can be readily controlled.