Provided is an adhesive composition including lysine, itaconic acid, and water, wherein the lysine and the itaconic acid are present in the form of an aqueous solution of a salt such that precipitates are not formed in the aqueous solution. In addition, provided is a method of preparing an adhesive composition including mixing lysine, itaconic acid, and water to thereby form a mixture of the lysine, the citric acid, and the water, and stirring the mixture at a temperature of 80° C. or less, wherein amounts of the lysine, the itaconic acid, and the water are adjusted such that precipitates of the lysine and the itaconic acid do not form in the adhesive composition.