Abstract: A thin film forming apparatus including a vacuum tank capable of introducing an active gas and/or an inert gas thereinto; a source of evaporable substance disposed in the tank; a counter electrode disposed opposite to the source of evaporable substance in the tank and holding a substrate on which a thin film is to be vapor-deposited; a filament disposed between the source of evaporable substance and the counter electrode for generating thermal electrons; a grid disposed between the filament and the source of evaporable substance, and allowing evaporated substance to pass therethrough; and power source means for putting the grid at a positive potential relative to the counter electrode. Electrons emitted from the filament form a plasma in the vicinity of the grid and ionize evaporable substance from the source of evaporable substance before being absorbed by the grid. The ionized evaporated substance then is accelerated toward the substrate for low-temperature deposition on the substrate.