A process for preparing a selective permeable membrane having self-supporting property, which comprises coating on a supporting substrate a dope comprising a polyimide polymer consisting essentially of a repeating unit of the formula: ##STR1## wherein R is a divalent organic group, an inorganic salt and an organic solvent, as a dope solvent, which dissolves a mixture of the polymer and the inorganic salt to form a homogeneous system, to prepare a dope coated supporting substrate, heat treating the substrate at a temperature which does not cause the organic solvent in the dope to boil, and then coagulating the substrate.