The present invention provides an ammonia oxidation/decomposition catalyst including a support composed of an oxidizable and reducible metal oxide and a catalytically active metal supported thereon. By bringing the ammonia oxidation/decomposition catalyst including a support composed of an oxidizable and reducible metal oxide and a catalytically active metal supported thereon into contact with ammonia and air at ordinary temperature, the support in a reduced state reacts with oxygen to generate oxidation heat, and the temperature of the catalyst layer is increased in a moment. Once the temperature of the catalyst layer is increased to a temperature at which ammonia and oxygen react with each other, the ammonia oxidation reaction proceeds autonomously after that. The heat generated in this exothermic reaction is used in the course of decomposing ammonia in the presence of the catalytically active metal, thereby producing hydrogen.