A method of preventing a lowering of concentration of a prostaglandin derivative, the prostaglandin derivative being 16-phenoxy-15-deoxy-15,15-difluoro-17,18,19,20-tetranorprostaglandin F2 .alpha. isopropyl ester, the prostaglandin derivative being contained in an ophthalmic solution as an active ingredient. The method including (i) adding to the ophthalmic solution a nonionic surfactant to inhibit the prostaglandin derivative from being adsorbed to a container which contains the ophthalmic solution, the container being made of a resin and (ii) adding to the ophthalmic solution an antioxidant to inhibit decomposition of the prostaglandin derivative.