PROBLEM TO BE SOLVED: To provide a method for manufacturing a coating liquid for forming a gas barrier layer capable, by conferring humidity resistance onto a film consisting of cellulose nanofibers, of manufacturing a gas barrier film which can preserve, even at a high humidity, the excellent gas barrier capacity of the cellulose film, can inhibit the invasion/osmosis, into the film, of steam, etc. serving as degradation factors, entails a scarce humidity-induced degradation of the film strength, and has a high adhesiveness.SOLUTION: The provided coating liquid for forming a gas barrier layer is manufactured by adding, to a cellulose nanofiber dispersion having undergone an oxidation treatment (S1) and a splitting treatment (S2), a water-soluble polymer, an inorganic laminar compound, and a sizing agent (S3). This manufacturing method, furthermore, comprises a step S10 of mixing an inorganic laminar compound with an aqueous water-soluble polymer solution and then agitating the obtained mixture; a step S20 of
