A drug for preventing or treating heart diseases which comprises an expression vector containing CD9 gene as the active ingredient. In the present invention, the term “heart diseases” means diseases causative of heart failure, ischemic heart diseases, cardiomyopathy, hypertensive heart diseases, valvular disease, congenital heart diseases, mayocarditis, arrhythmia and diseases associated with cardiac hypertrophy and / or tachycardia. In the present invention, the expression vector is a viral vector or a non-viral vector. A method for preventing or treating heart diseases which comprises expressing the CD9 gene in the heart. In the present invention, the prevention or the treatment is carried out by a gene therapy of transferring the CD9 gene. In the gene therapy, use is made of a drug comprising an expression vector containing the CD9 gene as the active ingredient.