Precise and the high-speed replacement of various gas species in the plasma process chamber become increasingly very important in order to realize high-quality continuous multi-processes using various gas species in a single plasma process chamber with high throughput in various industry manufacturing fields such as semiconductor devices, flat panel displays (FPD), solar cells and so on. In a microwave-excited high-density and low-electron temperature plasma process equipment with a dual-shower-plate structure, uniform down-flow of gas in the chamber is essential to realize this. Thus, the upper shower plate for down-flow supply of gas plays an important role. It has been confirmed that, in the case of the replacement of N2 by Ar, the short replacement time of around 2 seconds in the cases of 133 Pa, 66.7 Pa, 13.3 Pa and 6.67 Pa can be achieved by using the upper shower plate having 1200 holes. The upper shower plate has advantageous effect for high-speed gas replacement due to precise down-flow of gas.