Raw material gas supply nozzles are arranged within a virtual concentric circle having its center at the center of a disk-like base plate (having an area half as large as an area of the base plate). Raw material gas is ejected at a flow velocity of 150 m/sec or more into a bell jar from the gas supply nozzles. In addition to one gas supply nozzle provided in a center portion of the base plate, three gas supply nozzles can be arranged at the vertex positions of a regular triangle inscribed in a circumscribed circle having its center at the gas supply nozzle in the center portion. With the gas supply nozzles so arranged, a smooth circulating flow is formed within a reactor.