A process for producing a crosslinked type or non-crosslinked type layered metal phosphonate compound including a reaction step of reacting two or more members selected from organic diphosphonic acids or monophosphonic acids, or derivatives thereof having predetermined conditions and a metal source capable of forming an ion of a hexacoordinate metal atom as a central atom (M) of a metal oxide octahedron upon reaction under the presence of a sulfuric acid catalyst, a crosslinked or non-crosslinked metal phosphonate compound obtained by the process, as well as a stock solution used for the synthesis of the crosslinked or non-crosslinked type layered metal phosphonate compound described above.