The present invention provides a catalyst substance that is stable and performs well in the synthesis of ammonia, one of the most important chemical substances for fertilizer ingredients and the like. The catalyst substance exhibits catalytic activity under mild synthesis conditions not requiring high pressure, and is also advantageous from a resource perspective. Further provided is a method for producing the same. This catalyst comprises a supported metal catalyst that is supported on a mayenite type compound including conduction electrons of 1015 cm−3 or more and serving as a support for the ammonia synthesis catalyst. The mayenite type compound used as the support may take any form, including that of powder, a porous material, a sintered body, a thin-film, or a single crystal. Use of this catalyst makes it possible to increase the electron donating ability toward a transition metal.