A composite oxide including an oxide of a metal element L and an oxide of a metal element N, and represented by a composition of general formula LnN1-n, wherein the metal element L is a Group 1 element, a Group 2 element, or a Group 1 element and a Group 2 element, the metal element N comprises a Group 1 or Group 2 element other than the metal element L, n is 0.001 or more and 0.300 or less, the oxide of the metal element L and the oxide of the metal element N form no solid solution, and oxide particles of the metal element L are deposited on surfaces of oxide particles of the metal element N. Also, a metal-carrier material and an ammonia synthesis catalyst having, supported on this composite oxide, particles of at least one metal M selected from the group consisting of cobalt, iron, and nickel.