To greatly reduce damage of an object and prolong its life by performing plasma etching using an etchant including oxygen and at least one fluorine-including compound to remove a silicon-including coat film from the object.

The silicon-including coat film, formed by a low-pressure chemical vapor-phase deposition process (LPCVD), is removed by plasma etching using the etchant including oxygen and at least one fluorine-including compound from a boat in a diffusion furnace or a general object contaminated with the silicon-including coat film. The fluorine-including compound contains one or plurality of fluorocarbon hydrogen such as CF4, CHF3 and C2F6, or SF6, SiF4 or NF3. The concentration of the compound causing etching operation in the etchant and the ratio between the concentrations of the compound and the etchant are selected purposefully in accordance with the type and thickness of the etched coat film on the object to be cleaned.