The present invention provides a method for expanding and improving functional capacity of human adult-derived progenitor cells in vitro using a closed culture system. The present invention provides a favorable condition for cell therapy to promote tissue repair and organogenesis via vasculogenesis and angiogenesis in clinical settings. The proposed closed bag culture system for culturing hemangioblast comprises of, in one embodiment, a serum-free culture medium containing one or more factors selected from the group consisting of stem cell growth factor, interleukin-6, FMS-like tyrosine kinase 3, thrombopoietin, and vascular endothelial growth factor and a kit for the preparation of the serum-free culture medium and the like.