A 15-deoxyprostaglandin derivative of general formula (I) and an intraocular pressure depressant containing said derivative or a pharmaceutically acceptable salt thereof as the active ingredient. In formula (I), (a) is a five-membered ring selected from among (b), (c), (d), (e), (f); R¹ represents hydrogen or lower alkyl; and R² represents C₆ to C₁₂ alkyl, C₆ to C₁₂ alkenyl or C₆ to C₁₂ alkadienyl, provided that a compound of formula (II) (wherein R' represents H or CH₃) is excluded.