An object of the present invention is to provide a novel myocardial stem cell for transplantation which enables maintenance of a therapeutic effect on and/or preventive effect against cardiac failure for a prolonged period, a method for producing the myocardial stem cell, and a cell preparation containing the myocardial stem cell.[Solution]The present invention relates to a method for producing a myocardial stem cell to be used for treatment and/or prevention of cardiac failure, the method comprising the step of introducing a complex of a mitochondria-targeting carrier and a mitochondria activating agent into a myocardial stem cell； a cell produced by the method and a cell preparation containing the cell； and a liposome to be used for producing the cell. According to the present invention, there can be provided a myocardial stem cell which is capable of maintaining a therapeutic effect and/or preventive effect by cell transplantation for a prolonged period.
