PURPOSE: To simplify the constitution of the title optical CVD device by heating a substrate by photoirradiation process.

CONSTITUTION: A mixed gas of SiH4 in the film deposition rate selected at the composition ratio slower than the maximum rate is jetted from a gas feeder pipe 4 in A4 direction while another mixed gas of SiH4 and NH3 in the film deposition rate selected at the composition ratio almost equal to the maximum rate is jetted from another gas feeder pipe 5 in A5 direction. The light from an UV lamp 2 is irradiated in A3 direction through a quartz window 3 to give the photo energy to the mixed gas in a reaction vessel 1 while the mixed gas is cracked to be activated becoming an Si nitride which is deposited on wafers 7 making film deposition feasible. The film deposition rate is faster at the wafers 7 side and slower at the quartz window 3 side so that the films may be formed on the wafers while the quartz window 3 may not be blurred. Through these procedures, the constitution of the title optical CVD device can be simplified without assembling a heater into the reaction vessel.