PURPOSE: To make it possible to apply a reactive gas uniformly to a surface of a semiconductor wafer, by separating a first space, in which semiconductor wafers are arranged, and a second space, in which a reactive gas is supplied, and diffusing the gas through a porous plate from the second space to the first space.

CONSTITUTION: In a semiconductor manufacturing apparatus 20, a flange of quartz glass is formed on an inner circumferential face of an inner tube 3. A porous plate 22 having a plurality of through holes 23 of 1mm or above in diameter is fixed around a quartz cap 6. The flow of a reactive gas through a nozzle 7 into a double reactive tube 8 is blocked by the porous plate 22, and the reactive gas can not diffuse immediately into a wafer-boat arranged space 25A and remains still in a nozzle arranged space 25B. After a while, the reactive gas, which is increasing in the space 25B, diffuses into the wafer- boat 4 arranged space 25A uniformly through the through holes 23 of the porous plate 22. In this way, the reactive gas is applied uniformly to a wafer 5, and thereby the an insulating film is formed uniformly in thickness.