Nitrogen Fixation Systems(NFS) suitable for plant cultivation in a closed environment have been studied through experimental verification. The system is composed of physico-chemical processes only. Nitrogen gas and water are fed into the system as raw materials,and ammonia and ammonium nitrate solution are produced as final products, which are utilized as main fertilizers in plant cultivation. Each elemental chemical process in NFS is selected with regard to unique design criteria concerning safety, energy effectiveness, compactness and reliability. An experimental apparatus for important unit processes has been made for the purpose of verifying process data. A detail design for nitrogen fixation facilities, planned for construction as the first Japanese CELSS laboratory, was carried out.