PURPOSE: To make it possible to obtain an epitaxial growth film at a low temperature by a method wherein either thermal energy or thermal energy added with optical energy having a wavelength of 300nm or below are applied to a gas of chlorinated silicon containing Si2Cl6.

CONSTITUTION: Si2Cl6, as chlorinated silicon, is filled up in a bubbler 20 and vaporized by reducing the pressure inside the bubbler. With a lamp 4 for generating a wavelength of 300nm or below and an accompanying power source system 5 used for photochemical reaction, a lamp chamber 28 is connected to an exhaust system and a vacuum is drawn. A hydrogen gas is introduced into the lamp chamber 28 to prevent a back flow of a reactive gas, and moreover, the pressure in the lamp chamber 28 is adjusted by a valve 27 to be equal to that in a reaction chamber 1 so as not to prevent the breakdown of synthetic silica glass 26 of a window. This method enables the prevention of the absorption loss of ultraviolet rays, particularly the loss caused by oxygen in the atmosphere before the rays reach the inside of the reaction vessel 1 of shortwave beams.