PURPOSE: To remove the blood line of a large-sized fish such as tuna, and to prevent the discoloration of the meat, by rapidly and perfectly killing the fish by breaking its brain, etc., lowering the body temperature of the fish as rapidly as possible, and freezing or refrigerating the body for storage.

CONSTITUTION: Fished large-sized fish such as tuna is killed rapidly and perfectly e.g. by breaking its brain, and the body of the fish is cooled as rapidly as possible in a cooling water tank 1, and the fish is freezed or refrigerated for storage. A compressed air feed pipe 3 is placed near the cooling pipe 2 extended in the water tank 1 containing sea-water, and the pipe 3 is furnished with a number of air blowing holes 4 directed toward the cooling pipe 2. The sea-water in the water tank 1 is maintained at about 0°C by the cooling pipe, and a fish body subjected to the blood-removing treatment is left in the sea-water for several hours W ten hours to effect the above freezing or refrigeration treatment. The generation of the blood line in the fish meat can be suppressed remarkably, and the obtained meat is resistant to discoloration.