wafer in accordance with the change of the diameter d of the first reaction gas blowing region it can be appreciated from fig6 b that y t is constant if d is sufficiently greater than the wafer diameter however y t decreases suddenly due to dilution by n 2 if d becomes smaller it is to be noted that the decrease of y t does not commence just when diameter d becomes smaller than the wafer diameter and is substantially constant until approximately d 110 mm it can therefore be understood that the diameter may be reduced up to d 110 mm with respect to a wafer having a diameter of 150 mm to provide reaction gas uniformly to the surface of the wafer fig7 shows the thickness distribution of a cvd sio 2 film formed by emitting reaction gas a mixed gas of n 2 teos and o 3 and inert gas b n 2 respectively at an average flow rate of approximately 26 mm sec using the gas head of fig6 the vertical axis of fig7 represents the thickness of the sio 2 film it is appreciated from fig7 that the thickness of the cvd sio 2 film is substantially equal in above the central and the periphery.