A CVD device has a reaction furnace (39) for processing a wafer (1); a seal cap (20) for sealing the reaction furnace (39) hermetically; an isolation flange (42) opposite to the seal cap (20); a small chamber (43) formed by the seal cap (20), the isolation flange (42), and the wall surface in the reaction furnace (39); a feed pipe (19b) for supplying a first gas to the small chamber (43); an outflow passage (42a) provided in the small chamber (43) for allowing the first gas to flow into the reaction furnace (39); and a feed pipe (19a) provided downstream from the outflow passage (42a) for supplying a second gas into the reaction furnace (39). Byproducts such as NH4Cl are prevented from adhering to low temperature sections such as the furnace opening and therefore the semiconductor device production yield is therefore increased.