A gel sheet, prepared by immersing a primary hyaluronic acid gel sheet, which is prepared by drying a flat plate on which an aqueous solution obtained by dissolving an alkali salt of high-molecular-weight hyaluronic acid in an alkaline solution is applied in a vacuum oven, in a crude solution of a low-molecular-weight organic acid anhydride or an anhydride solution diluted with a unit organic acid forming the anhydride, and secondarily drying the immersed primary hyaluronic acid gel sheet in the vacuum oven.
