A method of manufacturing an adsorbent for carbon dioxide, including a composite metal oxide, which includes a divalent first metal (M 1), a trivalent second metal (M 2), and an element with an electronegativity ranging from 2.0 to 4.0 on the Pauling scale and has an amorphous structure, wherein the element is included in an amount of 0.3 moles to 3 moles per one mole of the sum of the divalent first metal M 1 and the trivalent second metal M 2, the method comprising: preparing a mixed aqueous solution by dissolving a salt of a divalent first metal and a salt of a trivalent second metal in water; adjusting pH of the mixed aqueous solution to be basic to precipitate a composite metal hydroxide; separating the composite metal hydroxide; mixing the composite metal hydroxide with an aqueous solution of a salt including an element with an electronegativity of 2.0 to 4.0 on the Pauling scale to obtain a mixture; adjusting pH of the mixture to be equal to or less than 7 and stirring the mixture to prepare an ion-exchanged composite metal hydroxide; separating the ion-exchanged composite metal hydroxide from the mixture; and calcining the ion-exchanged composite metal hydroxide to obtain a composite metal oxide, wherein the salts of the divalent first metal and of the trivalent second metal do not include a carbonate anion; wherein the divalent first metal (M 1) is selected from an alkaline-earth metal, a transition metal, and a combination thereof; wherein the trivalent second metal (M 2) is different from the divalent first metal (M 1) and is selected from a Group 13 element, a transition metal, a lanthanide, and a combination thereof; and wherein the element having an electronegativity ranging from 2.0 to 4.0 on the Pauling scale is selected from phosphorus, boron, fluorine, sulfur, chlorine, and a combination thereof. preparing a mixed aqueous solution by dissolving a salt of a divalent first metal and a salt of a trivalent second metal in water; adjusting pH of the mixed aqueous solution to be basic to precipitate a composite metal hydroxide; separating the composite metal hydroxide; mixing the composite metal hydroxide with an aqueous solution of a salt including an element with an electronegativity of 2.0 to 4.0 on the Pauling scale to obtain a mixture; adjusting pH of the mixture to be equal to or less than 7 and stirring the mixture to prepare an ion-exchanged composite metal hydroxide; separating the ion-exchanged composite metal hydroxide from the mixture; and calcining the ion-exchanged composite metal hydroxide to obtain a composite metal oxide, wherein the salts of the divalent first metal and of the trivalent second metal do not include a carbonate anion; wherein the divalent first metal (M 1) is selected from an alkaline-earth metal, a transition metal, and a combination thereof; wherein the trivalent second metal (M 2) is different from the divalent first metal (M 1) and is selected from a Group 13 element, a transition metal, a lanthanide, and a combination thereof; and wherein the element having an electronegativity ranging from 2.0 to 4.0 on the Pauling scale is selected from phosphorus, boron, fluorine, sulfur, chlorine, and a combination thereof.