The present invention provides an aqueous eye drop containing latanoprost, which is a preparation wherein decrease of an effective concentration of latanoprost due to adsorption to a resin is suppressed and the stability of latanoprost is improved. It is also an object of the present invention to provide a method of suppressing adsorption of latanoprost to a resin.The present invention provides an aqueous eye drop containing latanoprost, a surfactant, and an aliphatic mono- or di-carboxylic acid having a carbon number of 3 - 10 or a salt thereof. In addition, the present invention provides a method of suppressing adsorption of latanoprost to a resin in an aqueous solution, including adding a surfactant, and an aliphatic mono- or di-carboxylic acid having a carbon number of 3 - 10 or a salt thereof.
