This invention provides a cleaning method that uses a cleaning gas composition for a semiconductor manufacturing device, including a monofluorohalogen compound represented by XF (in which X is Cl, Br or I) as the main component, and provides a method for removing unwanted film, such as a Si-containing deposit, attached to the interior of the processing room or processing vessel after a processing operation without damaging the interior of the processing room or processing vessel using such monofluorohalogen compound.