A pyrazolone derivative represented by formula (I) below: wherein R1 to R3 are the same as defined in claims; or an optical isomer, a pharmaceutically acceptable salt thereof, or a hydrate or solvate thereof is provided. The novel pyrazolone derivative according to the present invention has a PAI-1 production inhibitory activity, a tissue fibrosis inhibitory activity, and a fibrolytic activity, and is effective for preventing and/or treating tissue fibrotic diseases (lung fibrosis, kidney fibrosis, etc.) and diseases of which a pathological thrombus becomes the cause, such as ischemic cardiac diseases (cardiac infarction and angina pectoris), atrial thrombus, lung embolism, deep thrombophlebitis, disseminated intravascular clotting, ischemic brain diseases (brain infarction, brain hemorrhage), and arterial sclerosis. In addition, a pharmaceutical agent for preventing and/or treating the disease conditions or the symptoms mediated by plasminogen activator inhibitor-1, comprising the novel pyrazolone derivative according to the present invention is also provided.