The present invention provides a stabilizer that can stabilize a salt of 10-(carboxymethylaminocarbonyl)-3,7-bis(dimethylamino)phenothiazine or a derivative thereof even under the existence of moisture or under light irradiation. A compound described in at least one of (1) and (2) below is used as the stabilizer of the salt of 10-(carboxymethylaminocarbonyl)-3,7-bis(dimethylamino)phenothiazine or the derivative thereof. (1) a surfactant having an alkyl group with a carbon number of 8 to 16 (2) at least one pigment substance selected from the group consisting of a compound represented by the following formula (I), a compound represented by the following formula (II), and a flavonoid pigment