The ammonia decomposition catalyst of the present invention is a catalyst for decomposing ammonia into nitrogen and hydrogen, including a catalytically active component containing at least one kind of transition metal selected from the group consisting of molybdenum, tungsten, vanadium, chromium, manganese, iron, cobalt, and nickel, preferably including: (I) a catalytically active component containing: at least one kind selected from the group consisting of molybdenum, tungsten, and vanadium; (II) a catalytically active component containing a nitride of at least one kind of transition metal selected from the group consisting of molybdenum, tungsten, vanadium, chromium, manganese, iron, cobalt, and nickel; or (III) a catalytically active component containing at least one kind of iron group metal selected from the group consisting of iron, cobalt, and nickel, and at least one metal oxide, thereby making it possible to effectively decompose ammonia into nitrogen and hydrogen at relatively low temperatures and at high space velocities to obtain high-pure hydrogen.