To obtain an inhibitor against change to black useful for prawns and crabs, comprising a hypochlorite, etc., and to provide a method for prevention treatment of change to black causing the same effect on prevention of change to black as that of a conventional method at a concentration lower than the conventional method.

An extracted solution is obtained by extracting a bamboo such as Phyllostachys pubescens, etc., or a part of Chikujyo with ethanol or a water-based extracting solvent such as a mixed solution of ethanol and water by a routine procedure. The extracted solution is treated with activated carbon and, if required, the activated carbon is removed. Then, if necessary, the treated extracted solution is formulated with the water-based extracting solvent such as water, ethanol, etc., to adjust a component concentration, formulated with various kinds of additives additive such as emulsifying agent, pH adjuster, stabilizer, antioxidant, etc., to give the objective auxiliary for preventing change to black. The auxiliary for preventing change to black is thrown in water or an aqueous solution of sodium chloride, having dissolved the inhibitor against change to black and sufficiently stirred to give a treating agent (liquid). Prawns/crabs are immersed in the treating agent (liquid) to carry out prevention treatment of change to black.