A method stabilizes pressure of a reaction chamber during a process using a first gas and a second gas, wherein a gas inlet line is connected to the reaction chamber, and a second gas line and a first gas line are connected to another end of the gas inlet line. The method includes: feeding a first gas in pulses according to a waveform to the reaction chamber through the first gas line and the gas inlet line; and feeding a second gas in pulses according to a reverse waveform to the reaction chamber through the second gas line and the gas inlet line, wherein superimposed waveforms of the waveform and reverse waveform are made substantially or nearly flat, thereby stabilizing pressure of the reaction chamber.