A method for stabilizing 10-(carboxymethylaminocarbonyl)-3,7-bis(dimethylamino)phenothiazine, or a salt thereof, the method comprising causing 10-(carboxymethylaminocarbonyl)-3,7-bis(dimethylamino)phenothiazine, or a salt thereof to be present with a stabilizer comprising: a surfactant having an alkyl group with a carbon number of 8 to 16; at least one pigment substance selected from the group consisting of a compound represented by the following formula, a compound represented by the following formula, and a flavonoid pigment: R 1 -N = N-R 2... where in the formula, R 1 is and R 2 is where in R 1 and R 2, X is hydrogen, halogen, sodium, or potassium, and the respective Xs may be identical to or different from each other, Y is hydrogen or SO 3 X, and the respective Ys may be identical to or different from each other, and Z is hydrogen, a methyl group, or a methoxy group, and the respective Zs may be identical to or different from each other, where in the formula, X is hydrogen, halogen, sodium, or potassium, and the respective Xs may be identical to or different from each other, and Y is hydrogen, halogen, sodium, or potassium, and the respective Ys may be identical to or different from each other; and quaternary ammonium having a hydrocarbon group with a carbon number of 12 or higher or a salt thereof. a surfactant having an alkyl group with a carbon number of 8 to 16; at least one pigment substance selected from the group consisting of a compound represented by the following formula, a compound represented by the following formula, and a flavonoid pigment: R 1 -N = N-R 2... where in the formula, R 1 is and R 2 is where in R 1 and R 2, X is hydrogen, halogen, sodium, or potassium, and the respective Xs may be identical to or different from each other, Y is hydrogen or SO 3 X, and the respective Ys may be identical to or different from each other, and Z is hydrogen, a methyl group, or a methoxy group, and the respective Zs may be identical to or different from each other, where in the formula, X is hydrogen, halogen, sodium, or potassium, and the respective Xs may be identical to or different from each other, and Y is hydrogen, halogen, sodium, or potassium, and the respective Ys may be identical to or different from each other; and where in the formula, R 1 is and R 2 is where in R 1 and R 2, X is hydrogen, halogen, sodium, or potassium, and the respective Xs may be identical to or different from each other, Y is hydrogen or SO 3 X, and the respective Ys may be identical to or different from each other, and Z is hydrogen, a methyl group, or a methoxy group, and the respective Zs may be identical to or different from each other, where in the formula, X is hydrogen, halogen, sodium, or potassium, and the respective Xs may be identical to or different from each other, and Y is hydrogen, halogen, sodium, or potassium, and the respective Ys may be identical to or different from each other; and X is hydrogen, halogen, sodium, or potassium, and the respective Xs may be identical to or different from each other, Y is hydrogen or SO 3 X, and the respective Ys may be identical to or different from each other, and Z is hydrogen, a methyl group, or a methoxy group, and the respective Zs may be identical to or different from each other, where in the formula, X is hydrogen, halogen, sodium, or potassium, and the respective Xs may be identical to or different from each other, and Y is hydrogen, halogen, sodium, or potassium, and the respective Ys may be identical to or different from each other; and X is hydrogen, halogen, sodium, or potassium, and the respective Xs may be identical to or different from each other, and Y is hydrogen, halogen, sodium, or potassium, and the respective Ys may be identical to or different from each other; and quaternary ammonium having a hydrocarbon group with a carbon number of 12 or higher or a salt thereof.