General formula (I): The present invention pertains to: a pyridone derivate or a salt thereof represented by general formula (I); or a medicine containing the pyridone derivative or salt thereof as an active ingredient. [In the formula, ring A, R 1 , R 2 , R 3 and R 4 are specific groups.]以一般式(I):〔式中,環A、R1、R2、R3及R4為特定的基〕所表示的吡啶酮衍生物或其鹽;或含有以前述吡啶酮衍生物或其鹽作為有效成分的醫藥品。