Abstract: The method for separating water by a separation membrane is a method for separating water from an aqueous organic acid solution by the separation membrane, and the separation membrane consists of polycrystalline membrane of mordenite (the chemical composition of the frame: AlnSi40-nO96, 2?n?, further, a major component of exchangeable cations present in ion exchange sites of the mordenite is protons (H+). The separation membrane for dehydrating an aqueous organic acid solution exhibits unexpectedly remarkable water permeability and has excellent acid resistance, without damaging water separation selectivity. The method for separating water using this separation membrane enables, for example, realizing the process of dehydrating acetic acid by the separation membrane, and a great energy saving. There are provided the above-described separation membrane for dehydrating an aqueous organic acid solution, and the method for manufacturing it.