Disclosed is an improvement of the chemical vapor deposition (CVD) process for preparing an electroluminescent device having a large area and a large size. The process includes forming a luminescent layer on a substrate by a reduced-pressure chemical vapor deposition method wherein a source material gas is introduced into a reaction chamber. The reaction chamber includes a screening means which screens the source material gas flow in the chamber to form a first area in which the gas is flowing and a second area in which the gas substantially does not flow. The screening means has apertures for connecting the first area and second area. the substrate is placed in the second area, to which the source material is supplied by means of gas diffusion.