PURPOSE: To prevent the occurrence of an eddy in the gas introducing section of a reaction furnace by providing a gas flow control mechanism on the gas introducing side of the, reaction furnace so that the gas flow introduced into the reaction furnace uniformly diffuses centering the vertex of a circular cone, thereby uniformly supplying the gas onto a crystal substrate.

CONSTITUTION: A gas flow control mechanism 20 is provided in the connecting section of a throttle section 12 of a reaction tube 11 and a gas introducing pipe 13. This gas flow control mechanism 20 is provided with a plurality of ports 21 for drawing out the gas on a circular arc centering the intersection point on the extension of the taper of the throttle section 12, or the vertex P of a circular cone making the throttle section 12, and having any radius from this center P. The gas introduced from the gas introducing pipe 13 diffuses along the taper of the throttle section 12 of the reaction tube 11 and flows uniformly by passing through the ports 21 for drawing the gas of the gas flow control mechanism 20. Further, if the holes outside the circular arc are made larger, the effect of the resistance of the flow due to the tube wall of the reaction tube 11 becomes small, whereby the gas can uniformly be supplied to a crystal substrate 15. Also, since the gas flow diffuses uniformly, the occurrence of an eddy in the shoulder of the reaction tube 11 can be prevented.