Abstract: A cleaning apparatus (30) is connected to a treating chamber (12) of a CVD apparatus (10) for forming a silicon film. The cleaning apparatus (30) has a first, a second, and a third gas sources (32, 34, 36) and a chlorine gas, a fluorine gas, and an inert gas are introduced from the gas sources through FMC (38a, 38b, 38c), respectively, with flow rates controlled independently from one another. Those gases are gathered at a pipe (42) and mixed into a mixed gas. The mixed gas is passed through a heated reactor (44) such as a heat exchanger to thereby react the chlorine gas with the fluorine gas and form a formed gas containing fluorinated chlorine gas such as CIF3. The formed gas is supplied to the treating chamber (12) through a cooler (46), an analyzer (48) and a buffer (54).