PURPOSE: To make removal of a reaction pipe for the purpose of cleaning easy and make the film thickness of the formed film uniform by inserting another one reaction pipe which is removable, to the inner side of the reaction pipe.

CONSTITUTION: In a diffusion furnace type CVD film forming apparatus consisting of a reaction pipe 1, a boat 2, silicon wafers 3, a gas inlet 5, an outlet 6 and mounting fittings 7, 8, a removable another one reaction pipe 9 is inserted into the reaction pipe 1. In the case of forming CVD films on the silicon wafers 3, the uniformity of the formed film thicknesses depend most largely upon the difference between wafer diameters and the inside diameter of the reaction pipe; therefore, the thicknesses of the formed films may be made uniform simply by replacing the inner reaction pipe 9 depending upon the sizes of the silicon wafers 3 and the need for cumbersome replacement of the reaction pipe 1 in the prior art may be eliminated. This apparatus is effective for the case of forming SiO2 films by combination of SiH4 and O2 particularly at low temperatures.