NOVELTY - The ion-conductive organic/inorganic composite membrane comprises a porous base material, and an ion-conductive inorganic material comprising phosphate platelets, a silicon compound and a Keggin-type polyoxometalate and Keggin-type heteropoly acid. The phosphate platelets are three-dimensionally connected to each other via the silicon compound, and have a thickness of 0.5-10 nm. The ion-conductive inorganic material is coated on a surface of the porous base material or impregnated throughout the entire porous base material.    USE - The ion-conductive organic/inorganic composite membrane of an electrolyte membrane is useful in a membrane electrode assembly and a fuel cell (all claimed).    ADVANTAGE - The ion-conductive organic/inorganic composite membrane has increased mechanical strength, and effectively prevents crossover of liquid fuel without the reduction of ion conductivity in the liquid fuel cell thus allowing for the production of fuel cells with excellent performance.    DETAILED DESCRIPTION - The ion-conductive organic/inorganic composite membrane comprises a porous base material, and an ion-conductive inorganic material comprising phosphate platelets, a silicon compound and a Keggin-type polyoxometalate and Keggin-type heteropoly acid. The phosphate platelets are three-dimensionally connected to each other via the silicon compound, and have a thickness of 0.5-10 nm. The ion-conductive inorganic material is coated on a surface of the porous base material or impregnated throughout the entire porous base material. A weight ratio of the phosphate platelets to the silicon compound is 1:0.2-1:2.5. A weight ratio of the phosphate platelets to the Keggin-type polyoxometalate and/or the Keggin-type heteropoly acid is 1000:1-1:1. The Keggin-type polyoxometalate is a salt formed by substituting protons of the Keggin-type heteropoly acid with alkali metal ions and/or ammonium ions. The ion-conductive inorganic material layer formed on the porous base material has a thickness of 0.2-10 mu m. An average diameter of pores in the porous base material is 0.05-10 mu m. The ion-conductive organic/inorganic composite membrane has a thickness of 50-200 mu m. INDEPENDENT CLAIMS are included for:    (1) a membrane electrode assembly; and    (2) a fuel cell.    DESCRIPTION OF DRAWING(S) - The diagram shows a scanning electron spectroscopy image of porous base materials for manufacturing ion-conductive organic/inorganic composite membranes.