A method for performing a process on a target in a chamber. A gas discharge unit includes a first space having a discharge hole for discharging a first gas, a second space having a discharge hole for discharging a second gas and a third space having a discharge hole for discharging a gas generated between the first and second spaces. A distribution unit includes a first distribution pipe communicating with the first space, a second distribution pipe communicating with the second space and a third distribution pipe communicating with the third space. A valve group includes a first valve opened or closed to the first distribution pipe and a second valve opened or closed to the second distribution pipe. The method includes switching, without mixing the first gas and the second gas, the gas discharged from the discharge hole in the third space by opening or closing the valve group.