Provided is a method for manufacturing a catalyst with which it is possible to obtain a supported metal ammonia synthesis catalyst, in which there are restrictions in terms of producing method and producing facility, and particularly large restrictions for industrial-scale producing, in a more simple manner and so that the obtained catalyst has a high activity. This method for manufacturing an ammonia synthesis catalyst includes: a first step for preparing 12CaO.7Al2O3 having a specific surface area of 5 m2/g or above; a second step for supporting a ruthenium compound on the 12CaO.7Al2O3; and a third step for performing a reduction process on the 12CaO.7Al2O3 supporting the ruthenium compound, obtained in the second step. This invention is characterized in that the reduction process is performed until the average particle diameter of the ruthenium after the reduction process has increased by at least 15% in relation to the average particle diameter of the ruthenium before the reduction process.