The present invention addresses the problems of providing a method for proliferating cardiomyocytes using a miRNA that promotes the proliferation of cardiomyocytes, a vector for use in the treatment of heart disease, a pharmaceutical composition for treating heart disease, and so forth.The present invention provides a method for proliferating cardiomyocytes using a miRNA having the cardiomyocyte proliferation promoting action, a vector for use in the treatment of heart disease that comprises said miRNA, a pharmaceutical composition for treating heart disease that comprises said vector, and so forth. A particularly preferred miRNA is one that is selected from the group consisting of mature miRNAs, i.e., miR-148a, miR-148b, miR-152, and miR-373, and precursors of said miRNAs, as well as variants and analogs thereof.
