Disclosed is an ammonia decomposition catalyst which is obtained by heat-treating a complex at a temperature of 360° C. to 900° C. in a reducing atmosphere, wherein the complex containing a polymer having a molecular weight of 1,000 to 500,000 represented by the formula [I], a transition metal coordinated with the polymer, and an activated carbon or a carbon nanotube added thereto. In a case of using the carbon nanotube, an alkali metal compound or an alkaline earth metal compound is added to the heat-treated complex. R1 represents H or hydrocarbon having 1 to 10 carbon atoms, R2 and R3 each represent H, halogen, nitro, acyl, ester, carboxyl, formyl, nitrile, sulfone, aryl, or alkyl group having 1 to 15 carbon atoms, X and Y each represent H or OH, Z represents CH or N, R4 and R5 each represent H, OH, ether, amino, aryl, or alkyl group having 1 to 15 carbon atoms, x represents a real number of 1 to 2, y represents a real number of 1 to 3, and n represents a real number of 2 to 120. The amount of the transition metal deposited on the catalyst can be increased without deteriorating the dispersion of the metal, so that the amount of the catalyst required to obtain a desired activity can be reduced.