Provided are: a composition that can impart a thermoplastic resin with excellent transparency and physical properties; a thermoplastic resin composition containing the same; and a molded article thereof. The composition contains: (A) a cyclic organophosphate aluminum salt represented by Formula (1) below, wherein R1 to R4 each independently represent a hydrogen atom or a linear or branched alkyl group having 1 to 9 carbon atoms, and R5 represents an alkylidene group having 1 to 4 carbon atoms; and (B) sodium carboxylate, wherein a molar ratio of the (A) cyclic organophosphate aluminum salt represented by Formula (1) and the (B) sodium carboxylate, (A)/(B), is in a range of 0.20 to 0.56.