PURPOSE: To produce plural species of mixed gases of different components in different mixing ratios by uniformly mixing plural species of component gases in optional ratios by a capillary type flow rate ratio mixing method.

CONSTITUTION: In plural places in the longitudinal direction of a mixig pipe 1, branched pipes 2AW2E for supplying the component gases, consisting of laminar flow elements, are radially arranged in groups of six in the circumferential direction. Since the flow rate in the branched pipe is so designed that it becomes larger toward the upstream of gas flow inside the mixing pipe 1, small amounts of component gases sent into the mixing pipe 1 through the branched pipes nearer to the outlet 1b of mixed gas are made to be directed to the way of the flow of component gases being sent in great amounts and therefore the variation in the mixing ratios with time can be suppressed. Also, since the direction of sending components gases through each branched pipe is intersected by he flow of gas sent through the mixing pipe 1, stirring actions caused by swirl flows are produced.