A prostate cancer progression inhibitor comprises 4-(4-cyano-2-{[2-(4-fluoro-1-naphthyl)propanoyl]amino}phenyl)butyric acid, a salt thereof, a solvate thereof, or a prodrug thereof. 4-(4-Cyano-2-{[2-(4-fluoro-1-naphthyl)propanoyl]amino}phenyl)butyric acid is useful as a prostate cancer progression inhibitor because this butyric acid has, for example, a growth inhibiting effect and a hormone responsiveness recovering effect on prostate cancer that has acquired hormone resistance.