To provide a new crosslinked layered phosphonic acid metal compound having high crystallinity, containing a multi-component phosphonic acid homogeneously introduced therein and free from fluorine atom, a method for producing the compound, a stock solution and a method for producing the solution.

The invention provides a method for producing a crosslinked layered phosphonic acid metal compound comprising the reaction of at least one kind of organic diphosphonic acid or its derivative satisfying a prescribed condition, at least one kind of a monophosphonic acid or its derivative and a metal source producing a six-coordinate metal atom ion giving the center atom (M) of an octahedral metal oxide in the reaction in the presence of a sulfuric acid catalyst, a crosslinked layered phosphonic acid metal compound produced by the method, a stock solution for the synthesis of the crosslinked layered phosphonic acid metal compound and a method for producing the solution.