Disclosed are: a method for proliferating cardiomyocytes wherein a miRNA that promotes the proliferation of cardiomyocytes is used a vector for the treatment of heart diseases and a pharmaceutical composition or the like for the treatment of heart diseases. Specifically disclosed are: a method for proliferating cardiomyocytes which uses a miRNA that has a cardiomyocyte proliferation promoting action a vector for the treatment of heart diseases which comprises the miRNA and a pharmaceutical composition or the like for the treatment of heart diseases which contains the vector. The miRNA is preferably one selected from the group consisting of miR 148a miR 148b miR 152 and miR 373 that are mature miRNAs precursors of the aforementioned miRNAs and mutants and analogues of the miRNAs and the precursors.
