At least once, air filling a piping from main hydrogen pipe line to an individual home end should be replaced with hydrogen gas to use the gas in the home. Special attention is required to complete the replacing operation safely, because air and supplied hydrogen may generate flammable/explosive gas mixture in the piping. The most probable method to fulfill the task is that, at first an inert gas is used to purge air from the piping, then hydrogen will be supplied into the piping. It is easily understood that the amount of the inert gas consumed by this method is much to purge whole air, especially in long piping system. Hence, to achieve more economical efficiency, an alternative method was considered. In this method, previously injected nitrogen between air and hydrogen prevents them from mixing. The key point is that how much the gas is required to prevent mixing and keep the condition in the piping safe. The authors investigated to find the minimum amount of the inert gas required to keep the replacing operation safe. The main objective of this study is to assess the effect of the nitrogen and estimate a pipe length that the safety is maintained under various conditions by using computational fluid dynamic (CFD). The effects of the amount of the injected nitrogen, the hydrogen-supply conditions and the structure of piping system are discussed.