A method for producing a solution containing at least an unsaturated carboxylic acid-modified vinyl alcohol polymer is characterized by reacting a vinyl alcohol polymer and an unsaturated carboxylic acid compound in the presence of an acid catalyst soluble in the reaction system or a solid acid catalyst, and a gas barrier film is obtained by using the same. An example thereof is a method for producing a solution (A) containing an unsaturated carboxylic acid-modified vinyl alcohol polymer (b), the method comprising following two steps of: (1) a reaction step in which the unsaturated carboxylic acid-modified vinyl alcohol polymer (b) is formed by reacting a vinyl alcohol polymer and an unsaturated carboxylic acid compound (a) in the presence of an acid catalyst soluble in the reaction system; and (2) an ion exchange step in which at least the acid catalyst of acidic compounds is removed by an anion exchange resin.