The use of ammonia as a fuel is expected to expand with the trends such as the commencement of demonstration of ammonia co-firing in the field of thermal power generation, and the development of ammonia-fueled engines in the field of marine vessels. The key factors for the demand expansion are considered to be the safety of operation because of the use of toxic ammonia, the reasonable economic efficiency as an alternative to fossil fuel, and the environmentally conservative properties involving no generation of greenhouse gases (GHGs) or by-products that can lead to environmental destruction. The ammonia decomposition system, which is proposed by Mitsubishi Heavy Industries, Ltd. (MHI), employs its original catalytic technology and will be one of the means to satisfy these factors. However, the operation conditions of such systems and the requirement specifications have yet to be clarified. This report summarizes our system that can satisfy the factors, in addition to the specifications and features. Even in a project requiring the reduction of NOx by-products in the exhaust gas after ammonia is decomposed, the increase in the system size can remain at a minimum. Our system was also development designed in modules to make it transportable. We hope that this report will become a proposal for customers who are considering the use of ammonia as a fuel, and will lead them to determine their own system operation conditions and specifications required for ammonia gas or wastewater detoxification.