This invention relates to a novel thermoplastic and amorphous polyimide which is readily soluble in organic solvent, a novel aromatic diamino compound used for the polyimide, a preparation process thereof, a polyimide-based resin composition comprising the thermoplastic polyimide and a fibrous reinforcement, a process for preparing the resin composition, an injection molded article of the resin composition, a polyimide-based composite obtained by molding the above soluble and thermoplastic polyimide in combination with a fibrous reinforcement, and a fibrous reinforcement having a surface modified with the above polyimide. The polyimide comprise a requisite structural unit having one or more recurring structural units of the formula: ##STR1## wherein m and n are individually an integer of 0 or 1, and R is ##STR2## The polyimide of the invention is essentially amorphous, excellent in melt flow stability in the decreased temperature as compared with conventionally known polyimide and has greatly improved processability. The polyimide of the invention obtained by using a novel aromatic diamino compound as a monomer can control various properties such as melt flow ability and solubility in solvents by side chains and not by principal chain of polyimide. Thus, excellent melt flow property and solubility in solvents can be obtained while maintaining high heat resistance and adhesive property which are derived from benzophenone structure of the diamino compound of the the invention.