Disclosed is an external composition usable as an external skin preparation or a cosmetic which has an excellent effect of removing active oxygen, a tyrosinase inhibitory effect and a reducing effect and exerts an antiaging function based on the antioxidative effect and a whitening function based on the tyrosinase inhibitory effect, a hair cosmetic which prevents hair from damage, and a perming agent, a curling agent, an unruly hair smoothening agent, a hair dye and so on which are based on the reducing effect on disulfide bond. An external composition characterized by comprising one or more members selected from among 2-thiothiazolidine-4-carboxylic acid represented by chemical formula 1 or a derivative thereof and 2-methylthiazolidine-2,4-dicarboxylic acid represented by chemical formula 2 or a derivative thereof or a salt of the same.
