The present invention relates to endocardium-derived adult stem cells obtained by culturing peripheral blood mononuclear cells (PBMCs) separated from peripheral blood, and a cell therapeutic agent for treating cardiovascular diseases containing the same as an active ingredient. The adult stem cells, according to the present invention, have an origin that is the endocardium and strong blood vessel formation, and is thus remarkably useful for treating cardiovascular diseases such as ischemia, myocardial infarction, and the like.