invention will be described in detail with reference to a presently preferred embodiment thereof as illustrated in fig2 in fig2 the same reference numerals as in fig1 indicate the same or corresponding parts in fig2 an internal vessel 94 is disposed in a vacuum chamber 6 in a spaced opposite relation to a substrate 7 a gas ion source 9 for activating namely exciting dissociating or ionizing reactive gases in the neighborhood of the substrate 7 is provided which comprises an internal vessel 94 and several members within the vessel 94 which will be described in detail later the reactive gases may be at least one of oxygen nitrogen and hydrogen or may contain elements which produce compounds of a thin film to be deposited on the substrate 7 the reactive gases are introduced into the vacuum chamber 6 through a gas introduction means 4 and injected therefrom into the internal vessel 94 from a gas injection nozzle 44 the gas injection nozzle 44 is connected with the inner end of a pipe 43 which extends through and is hermetically mounted on a side wall of the chamber 6 through an insulating member 45 formed of ceramic an electron beam emitting means 92 in the form of a filament is disposed around a path of the reactive gases which are injected from the nozzle 44 an electron beam extraction electrode 91 is also disposed in and around the path of the injected reactive gases for extracting.