Multiprocesses in a single plasma process chamber with high throughput require precise, sequential, high-speed alteration of partial pressures of multiple gas species. A conventional gas-distribution system cannot realize this because the system seriously overshoots gas pressure immediately following valve operation. Furthermore, chamber volume and conductance of gas piping between the system and chamber should both be considered because they delay the stabilizing time of gas pressure. Therefore, the authors proposed a new gas-distribution system without overshoot by controlling gas flow rate based on pressure measurement, as well as a method of pulse-controlled gas injection immediately following valve operation. Time variation of measured partial pressure agrees well with a calculation based on an equivalent-circuit model that represents the chamber and gas piping between the system and chamber. Using pulse-controlled gas injection, the stabilizing time can be reduced drastically to 0.6 s for HBr added to pure Ar plasma, and 0.7 s for O-2 added to Ar/HBr plasma; without the pulse control, the stabilizing times are 3 and 7 s, respectively. In the O-2 addition case, rapid stabilization can be achieved during the period of line/space pattern etching of poly-Si on a thin SiO2 film. This occurs without anomalous etching of the underlying SiO2 film or the Si substrate near the sidewall, thus obtaining a wide process margin with high throughput.