In an aqueous composition comprising a 15-keto-prostaglandin compound as an active ingredient, a polyoxyethylene castor oil derivative is admixed. By admixing a polyoxyethylene castor oil derivative, the solubility of the 15-keto-PG compound in water is increased. In a preferable embodiment, the aqueous composition may further comprise a quaternary ammonium type cationic surface active agent as a preservative. The composition of this embodiment exhibits high antimicrobial preservative effectiveness against indigenous bacterium in the eyes such as Staphylococcus aureus and therefore, the composition is especially useful as ophthalmic composition for ocular topical administration.