A vertical furnace for use in a semiconductor manufacturing apparatus, which comprises a heater, an outer tube, an inner tube, all being disposed concentrically in a multi-layered fashion, a boat adapted to be introduced into the inner tube with a wafer loaded thereon, and a boat cover disposed internally of the inner tube concentrically therewith. The boat cover is comprised of a boat cover body and an auxiliary cover plate connected to said boat cover body with a given gap therebetween, the boat cover body having a predetermined number of slit apertures extending in a generator direction thereof, the auxiliary cover plate being disposed to cover the slit apertures. The introduced reactive gas flows in branched streams, one flowing through the inside of the boat cover and the other flowing in past the boat cover, whereby the film deposited on the wafer is improved in uniformity and homogeneity. Further, since the boat cover is provided on the inner tube in the form of a unitary body, adjustments relative thereto can be made easily, thus improving the efficiency of the maintenance works thereof.