Membrane detritiation processes using gas permeation membrane have the advantages of being simple in system configuration and compact in size compared to conventional oxidation followed by adsorption process. The experiment was performed using a gas separation module made of a polyimide membrane with dry nitrogen gas containing small amount of hydrogen gas and moisture, and an analytical study was performed to establish the theoretical treatments of the membrane separation of hydrogen, water from nitrogen. The experimental and analytical study revealed that the membrane separation process has a high selectivity for hydrogen and water from nitrogen, especially separation performance for water is very high. The effect of operation parameters on the separation performance can be predicted, and the design of the detritiation system of a fusion reactor facility can be constructed based on the present study.