Abstract: A method for processing a fish of red flesh includes a packing step of cutting a fish in such a shape as would fit immediate use by consumers, piling the cut fish orderly on a tray and stowing the cut fish on the tray into a pack, a vacuumizing step of filling the pack during the packing step with a vacuum atmosphere, a gas-filling step of filling the pack immediately after completion of the vacuumizing step with a mixed gas of carbonic acid gas and oxygen gas without exposing the interior of the pack to the ambient air, thereby exposing the fish to the mixed gas, a pack-sealing step of tightly closing the pack filled with the carabonic acid and the oxygen gas, and a freezing step of quickly freezing the fish contained in the pack and transforming the fish into a frozen fish.