A thin film deposition system includes an evacuated casing in which there is introduced an active gas or an inert gas or a mixture thereof. An evaporant carried by an evaporation source in the evacuated casing is evaporated and travels toward a substrate supported on the electrode. A filament for emitting thermions is disposed between the evaporation source and the electrode, and a grid for passing the evaporated material therethrough is disposed between the filament and the electrode. Power supplies are electrically connected to the evacuated casing, the evaporation source, the electrode, the filament, and the grid, for keeping the filament negative in potential with respect to the evacuated casing and the grid. Alternatively, the evacuated casing may not be connected to the power supplies so that the grid is kept positive with respect to the filament. The grid may be of a double-layer structure, with the gas inlet means connected to the grid.