A catalyst is provided which is used for continuously synthesizing ammonia using a gas containing hydrogen and nitrogen as a raw material, wherein a transition metal which exhibits catalytic activity is supported by a support, and the support is a two-dimensional electride or a precursor thereof. The two-dimensional electride or the precursor thereof is a metal nitride represented by MxNyHz (M represents one or two or more of Group II metals selected from the group consisting of Mg, Ca, Sr and Ba, and x, y and z are in ranges of 1≤x≤11, 1≤y≤8, and 0≤z≤4 respectively, in which x is an integer, and y and z are not limited to an integer) or M3N2 (M is the same as above), or a metal carbide selected from the group consisting of Y2C, Sc2C, Gd2C, Tb2C, Dy2C, Ho2C and Er2C. These catalysts are used for continuously reacting nitrogen with hydrogen, which are raw materials, on the catalyst, wherein the reaction is performed in an ammonia synthesis reaction system under the preferable conditions of a reaction temperature which is equal to or higher than 100° C. and equal to or lower than 600° C., and a reaction pressure which is equal to or higher than 10 kPa and lower than 20 MPa.