Substrate-processing apparatus and method of producing a semiconductor device

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 ( 19 b) for supplying a first gas to the small chamber ( 43 ); an outflow passage ( 42 a) provided in the small chamber ( 43 ) for allowing the first gas to flow into the reaction furnace ( 39 ); and a feed pipe ( 19 a) provided downstream from the outflow passage ( 42 a) 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.