A water-treatment separating membrane comprising: a polyamide active layer formed on a porous support; wherein the polyamide active layer is formed by dropping droplets of an organic solution including an acyl halide compound on an aqueous amine solution layer, where the polyamide active layer has center portions and critical areas corresponding to the center portions and peripheral portions of the droplets formed during planarization of the droplets, respectively, where the degree of interface polymerization with an amine compound at the center portions is higher than the degree of interface polymerization with the amine compound at the critical areas and the thickness of a polyamide active layer decreases from the center portions to the critical areas wherein a ratio of a maximum value with respect to a minimum value of a ratio of a peak height of a C=O double bond of a carboxyl group (-COOH) with respect to a peak height of a C=O double bond combined with an amide group (-CONH-) is from 1.2 to 4, wherein the peak height values are determined using Fourier transform infrared (FTIR) measuring and by using an FTIR spectrometer after acid treating the polyamide active layer, wherein the minimum value is the ratio of the peak height of the C=O double bond of the carboxyl group (-COOH) with respect to the peak height of the C=O double bond combined with the amide group (-CONH-) in the center portion, and the maximum value is the ratio of the peak height of the C=O double bond of the carboxyl group (-COOH) with respect to the peak height of the C=O double bond combined with the amide group (-CONH-) in the critical area, wherein the acid treating is performed by immersing the polyamide active layer in an aqueous hydrochloric acid solution, wherein the distance between the center portions is from 1 mm to 50 mm. a polyamide active layer formed on a porous support; wherein the polyamide active layer is formed by dropping droplets of an organic solution including an acyl halide compound on an aqueous amine solution layer, where the polyamide active layer has center portions and critical areas corresponding to the center portions and peripheral portions of the droplets formed during planarization of the droplets, respectively, where the degree of interface polymerization with an amine compound at the center portions is higher than the degree of interface polymerization with the amine compound at the critical areas and the thickness of a polyamide active layer decreases from the center portions to the critical areas wherein a ratio of a maximum value with respect to a minimum value of a ratio of a peak height of a C=O double bond of a carboxyl group (-COOH) with respect to a peak height of a C=O double bond combined with an amide group (-CONH-) is from 1.2 to 4, wherein the peak height values are determined using Fourier transform infrared (FTIR) measuring and by using an FTIR spectrometer after acid treating the polyamide active layer, wherein the minimum value is the ratio of the peak height of the C=O double bond of the carboxyl group (-COOH) with respect to the peak height of the C=O double bond combined with the amide group (-CONH-) in the center portion, and the maximum value is the ratio of the peak height of the C=O double bond of the carboxyl group (-COOH) with respect to the peak height of the C=O double bond combined with the amide group (-CONH-) in the critical area, wherein the acid treating is performed by immersing the polyamide active layer in an aqueous hydrochloric acid solution, wherein the distance between the center portions is from 1 mm to 50 mm.