Development of the Nitrogen Fixation System which is one of the subsystem of Closed Ecological Life Support System(CELSS) is necessary for the purpose of synthesizing nitrogen fertilizer, ammonia and ammonium nitrate, for plant cultivation in the closed system.Up to now, nitrogen fixation process has been established as ammonia synthesis using nitrogen and hydrogen in the industrial field, and ammonia is further oxidized to obtain the nitrate to be used for plant cultivation. The nitrogen fixation process consists of the subsystems for ammonia synthesis, ammonia separation, and nitric acid synthesis. Nowadays, this nitrogen fixation system has been put to practical use in the large commercial plant, however the system for CELSS is still developing technology. Considering such particular condition as for CELSS, it is needed for the total system to satisfy some important requirements such as safety, energy saving, compactness, maintainability and so on.In this paper, we discuss the practical system configuration for CELSS with the subsystems of low pressure ammonia synthesis using ruthenium (Ru) catalyst, ammonia separation and nitric acid synthesis with platinum (Pt)-rhodiam (Rh) catalyst. The basic experimental test results on ammonia synthesis are also discussed. The ammonia synthesis tests are conducted under 0- 1. OMPa and 573-673 K. These results show the realizability of the proposed nitrogen fixation process for CELSS.