A process includes the steps of: casting or coating a polyamic acid organic solvent solution on a support and drying the polyamic acid organic solvent solution thereon, so as to form a partially cured and/or partially dried polyamic acid film; dipping the polyamic acid film in tertiary amine or a solution of tertiary amine, or coating tertiary amine or a solution of tertiary amine on the polyamic acid film; and drying the film while imidizing the polyamic acid. In another process, a chemical converting agent and a catalyst are mixed in an organic solvent solution of polyamic acid. After casting and heating the mixture on a support, a partially cured and/or partially dried polyamic acid film is detached from the support. The film contains, with respect to the remaining volatile component, not less than 50 parts of catalyst, not more than 30 parts of solvent, and not more than 20 parts of chemical converting agent and/or a chemical converting agent derived component. The remaining amic acid is imidized and the film is dried.