PURPOSE: To obtain a thin film of polymer excellent in heat resistance, adhesiveness, etc., by evaporating specified vaporizable polymeric and/or polymerizable compound in a vacuum and reacting the formed vapors.

CONSTITUTION: Evaporation tubes 6 and 9 charged with a bismaleimide of formula I [wherein Ar is a group of formula II or III (wherein X is -O-, -S-, -SO2-, -NH-, -CO-, -CS-or a 1-4C alkylene; R1-2 are each H, a halogen atom, an OH group, a 1-4C alkyl or a 1-4C alkoxy; and n and m are the numbers of the substituents R1-2 are each 1-4)] and vaporizable polymeric and/or polymerizable compounds are set in a vacuum chamber 1, and the chamber 1 is evacuated to 1×10-4Torr or below by operating a vacuum pump 12 while the evaporation tubes 6 and 9 are heated by applying a current to heaters 7 and 10 to evaporate the bismaleimide and the polymer. These vapors are allowed to deposit on a substrate 2 mounted on a substrate holder 3 and reacted with each other by heating with a heater 4 to form deposited film of a crosslinked bisimide polymer.