The present inventors have found that an HMGB1 fragment peptide having a specific amino acid sequence exhibits an effect to improve the function of the heart, prevent cardiomyocyte hypertrophy, prevent myocardial fibrosis and promote angiogenesis in an animal model with dilated cardiomyopathy, the specific HMGB1 fragment peptide also exhibits an effect to improve the function of the heart, prevent auxocardia, prevent cardiomyocyte hypertrophy, prevent myocardial fibrosis and promote angiogenesis in an animal model with ischemic cardiomyopathy associated with old myocardial infarction, and the specific HMGB1 fragment peptide exhibits an effect to prevent cardiomyocyte hypertrophy and prevent myocardial fibrosis in an animal model with hypertensive cardiomyopathy. On the basis of this finding, a pharmaceutical composition for preventing and/or treating cardiomyopathy, old myocardial infarction, and chronic heart failure associated with these diseases is provided, which contains an HMGB1 fragment peptide having a specific amino acid sequence.
