The present invention relates to a catalyst for ammonia decomposition which is a catalyst for decomposing ammonia into nitrogen and hydrogen. The catalyst includes an active catalyst ingredient which comprises at least one transition metal selected from a group consisting of molybdenum, tungsten, vanadium, chromium, manganese, iron, cobalt, and nickel. It is preferred that (I) the active catalyst ingredient comprise at least one member selected from a group consisting of molybdenum, tungsten, and vanadium, or (II) the active catalyst ingredient comprise a nitride of at least one transition metal selected from a group consisting of molybdenum, tungsten, vanadium, chromium, manganese, iron, cobalt, and nickel, or (III) the active catalyst ingredient contain at least one iron-family metal selected from a group consisting of iron, cobalt, and nickel and further contain a metal oxide. This catalyst enables ammonia to be efficiently decomposed into nitrogen and hydrogen at a relatively low temperature and a high space velocity, whereby high-purity hydrogen is acquired.