A flow rate ratio control device having: a main channel (RM); branch channels (R1, R2) branching from the end of the main channel (RM); control valves (31, 32) respectively provided in the branch channels (R1, R2); flow rate measurement means for measuring the flow rate of the main channel (RM) and the flow rate of each of the branch channels (R1, R2); a valve control section (5) for controlling the a control valve (31) provided in one branch channel (R2) so that the pressure on the upstream side of the control valve (31) is a given target pressure and, only when the upstream-side pressure is near or not less than the target pressure, starting control of the other control valve (32) so that the flow rate ratios of the branch channels (R1, R2) are predetermined ratios; and a target pressure setting section (6) for setting the target pressure such that the smaller the flow volume is in the main channel (RM), the lower is the target pressure.