An apparatus for mixing and feeding plural gases flowing at different mass flow rates and having different molecular weights includes a plurality of gas feed lines connected to a mixing region having an outlet for feeding a mixture of the gases to a semiconductor production apparatus. The gas feed line carrying the lowest-flow-rate gas is connected to the mixing region at a location farther from the outlet than where any other feed line is connected to the mixing region. Feed lines carrying gases other than the lowest-flow-rate gas are connected to the mixing region according to (1) the relative mass flow rates of the gases carried by the lines (2) the relative molecular weights of the gases carried by the lines or (3) the product of the respective gas flow rates and molecular weights of the gases. At least the line carrying the lowest-flow-rate gas is provided with an apparatus for increasing the velocity of the gas flowing therein prior to entry of the gas into the mixing region.