The gelatin sol obtained by heating to dissolve amply, preferably with degassing, is then extruded into a coagulating solution through a spinneret to be formed by spinning into a streamlined shape. The spinneret may have an aperture of about 0.5 to 5.0 mm in diameter, preferably about 1.0 to 3.0 mm, corresponding to the thickness of shark fin. As the coagulating solution, an aqueous solution of a divalent or trivalent metal salt is used. The divalent or trivalent metal salt is preferably a salt of calcium, magnesium, aluminum or iron. Among these salts, an aqueous solution of a salt of calcium or aluminum, for example, calcium chloride, calcium hydroxide, or aluminum sulfate, is preferable. While the concentration of the aqueous solution depends on the salt employed, a concentration of the order of 1 to 10% is preferred.