A composite oxide for a hydrocarbon reforming catalyst which maintains the catalytic activity at a high level over a long period of time, a process for producing the catalyst, and a process for producing syngas using the catalyst, are provided. The composite oxide for a hydrocarbon reforming catalyst is obtained by a process including preparing a mixed solution for impregnation which contains catalytic active components of Co, or Co and Ni, one or more oxidation resistance enhancing components selected from the elements of Group 3B and the elements of Group 6A of the Periodic Table, and one or more additive metal components selected from Ca and Mg; impregnating a carrier formed from a porous molded body selected from magnesia and a composite of magnesia and calcia, with the mixed solution for impregnation; drying the impregnated carrier; and calcining the dried carrier in an oxidizing atmosphere.