A method of forming by CVD technique a layer of material with good uniformity and reproducibility on the surfaces of a plurality of substrates supported within the reaction chamber. The feature of the invention is that a gaseous mixture containing a reaction gas is supplied into the reaction chamber from the inlet of the reaction chamber and the auxiliary gas nozzle provided between the inlet and the exhaust in a predetermined control manner. Moreover, part of the gaseous mixture within the reaction chamber is sampled from the gas flow for the measurement of the concentration of the reaction gas, and from the measured results is determined the rate of gas supply from the auxiliary gas nozzle.