PURPOSE: To prepare a polymer having excellent heat resistance and useful as an electrical insulation material, molding material, etc., by polymerizing a specific polyimide compound with a polybenzimidazole compound.

CONSTITUTION: The objective polymer is obtained by reacting (A) 95W50mol% of a polyimide compound having at least two groups of formulaI(D is bivalent group having C=C group) in one molecule, especially a polyimide compound wherein D is group of formula II with (B) 5W50mol% of a polybenzimidazole compound having at least two groups selected from the group of formula III (R and R' are H, 1W20C hydrocarbon group, halogen, or nitro) and the group of formula IV (R" is same as R) in one molecule (e.g. 5,5'-dibenzimidazolylmethane, etc.) at 50W150°C in the presence of a solvent or at 100W200°C in the absence of a solvent, for 0.1W30hr. When the polymerization is carried out in a solution, the product can be used as a varnish as it is, and the powder of the polymer can be formed to a molded article by hot-pressing in a mold.