A first tank 12 accommodating the sodium bicarbonate 11 and a second tank 15 accommodating the combustion material 14 are separately provided in the middle of the gas passage 8 . When the compressed air A flows through the gas passage 8 toward the injection nozzle 2, first, the sodium carbonate 11 of the first tank 12 is supplied to the compressed air A, and then the second tank ( The combustion material 14 of 15) is supplied to the compressed air (A). Thereby, the sodium bicarbonate 11 and the combustion material 14 are mixed in the compressed air A at a predetermined ratio, and the compressed air A is mixed with the high-pressure water 3 in the injection nozzle 2, It becomes the mixed air jet J, and is sprayed on the outer surface Wa of the hull W. Since there is no non-uniformity in the ratio of the sodium bicarbonate 11 and the combustion material 14 contained in the mixed gas jet J, when the coating film of the outer surface Wa of the hull W is peeled off, the mixed gas jet J is injected. While being able to peel a coating film efficiently without non-uniformity with respect to the whole area of a location, a high rust prevention effect can be acquired.
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