PURPOSE: To form a uniformly deposited film on a substrate to be treated with the good efficiency of using a gaseous raw material by providing a gas introducing part of a thin film deposition device using a reactive ion beam to an electrode of an ion beam decelerating lens system near the substrate to be treated.

CONSTITUTION: The 1st, 2nd and 3rd electrodes 3, 3, 2 are disposed in a vacuum vessel 6 and the electrode 1 is in common use as a susceptor for the semiconductor substrate 10 to be formed with a thin film consisting of SiO2, etc. The substrate 10 is heated to a prescribed temp. by a heater 7. A pipe 4 for introducing a reactive gas such as oxygen is united to the 2nd electrode 2 and 16W32 pieces of gaseous oxygen ejection nozzles are preliminarily provided to the surface thereof. Respectively 0V,-2kV, and 24.5kV voltages are impressed to the electrodes 3, 2, 1 and the oxygen is ejected from the many nozzles of the electrode 2. The Si semiconductor substrate 10 on the electrode 1 via the electrodes 2, 3 and at the same time the gaseous molecules near the substrate are adsorbed by a cooling pipe 5 using liquid nitrogen as a refrigerant to increase the degree of vacuum and to concentrate the gaseous oxygen to the surface of the substrate 10, by which the uniform SiO2 thin film is formed on the substrate at a good raw material yield.