PROBLEM TO BE SOLVED: To effectively prevent color deterioration of dark flesh of fish in a short time when a fish such as Seriola quinqueradiata or tuna is cut.SOLUTION: The motor function of a fish such as a Seriola quinqueradiata Japanese amberjack is inhibited by damaging its spinal cord. After inserting an injection needle 5p of a syringe 5 into a bulbus arteriosus 14 of a heart 10 of the fish and withdrawing a part of its blood, a color tone maintaining liquid containing components of vitamin C and vitamin E that is nearly equal to the amount of the withdrawn blood. After that, the fish is put back to a tank for a rest. Operations are easier if a tubular insertion aid 4 which assists insertion of the injection needle 5p is used when the syringe 5 for withdrawing the blood or injecting the color tone maintaining liquid is operated.
