To find a compound having a tissue fibrinosis-inhibitory activity and a fibrinolytic activity, and to provide a novel compound that is useful for preventing and/or treating tissue fibrinosis diseases (pulmonary fibrosis, renal fibrosis etc.), diseases caused by pathological blood clots such as ischemic heart diseases (myocardial infarction, angina), intraatrial thrombus, pulmonary embolism, deep venous thrombosis, disseminated intravascular coagulation, ischemic cerebral diseases (cerebral infarction, cerebral bleeding) and arteriosclerosis and the like. To provide a pharmaceutical drug comprising a 5-phenyl-3-pyridazinone derivative represented by the following general formula (I): and an optical isomer thereof, or a pharmaceutically acceptable salt thereof, or a hydrate or solvate thereof, useful for preventing and/or treating disease conditions or symptoms mediated by plasminogen activator inhibitor-1.