The present invention relates to a processing apparatus for performing a film-deposition process or the like for an object to be processed (such as a semiconductor wafer) by means of a source gas and a reactive gas. The processing apparatus includes: a processing vessel (22) that contains therein an object to be processed (W); a source gas supply system (50) that selectively supplies a source gas into the processing vessel; a reactive gas supply system (52) that selectively supplies a reactive gas into the processing vessel; and a vacuum evacuating system (36) having vacuum pumps (44, 46), the system exhausting an atmosphere in the processing vessel to form a vacuum. The processing apparatus further includes: a source gas by-pass line (62) that selectively feeds the source gas into the vacuum evacuating system with the source gas bypassing the processing vessel; and a reactive gas by-pass line that selectively feeds the reactive gas to the vacuum evacuating system with the reactive gas bypassing the processing vessel. The source gas by-pass line (62) has a source gas escape prevention valve (X1) that prevents an escape of the source gas from the by-pass line into the vacuum evacuating system when the valve is in the closed condition. The reactive gas by-pass line (66) has a reactive gas escape prevention valve (Y1) that prevents an escape of the reactive gas from the by-pass line into the vacuum evacuating system when the valve is in the closed condition.