A material gas (source gas) containing silicon (such as an organic silicon gas expressed by at least Si??O??CxHy wherein ?? is an integer of >0 and ??, x and y are integers of >=0) and added gases such as N2O, He and Ar are controlled at a stated flow through feeding devices 6-8 and flow regulators 9-11, and after these gases are mixed, they are brought in an inlet 11 at the top of the upper elect