PURPOSE: To provide a photo-CVD apparatus suitable for preventing the clogging of a gas blow-off hole and a gas supply pipe of reaction gas even when film forming is continued for a long time.

CONSTITUTION: Gas supplied to a reaction chamber 1 is irradiated with light from a light source 3 through a light inputting window 2 to accelerate chemical reaction, causing a thin film to be deposited on a substrate 4. Reaction gas which can cause a clouding of the windowpane as a simple substance, e.g. monosilane A diluted with gaseous nitrogen is supplied from a gas supply pipe 6 through a gas blow-off hole 8 consisting of a perforated plate. Reaction gas which does not cause a clouding of the windowpane as a simple substance, e.g. oxygen B diluted with gaseous nitrogen is supplied from a gas supply pipe 7 though a gas blow-off hole 9 consisting of a perforated plate. A partition plate 10 projected from between the gas blow-off holes 8, 9 to inside the reaction chamber 1 is provided to prevent gas counter diffusion near the gas blow-off holes 8, 9.