The present invention provides a method for artificially producing a vascular endothelial stem cell from a non-vascular endothelial stem cell. The method for producing a vascular endothelial stem cell of the present invention comprises the step of bringing a vascular endothelial cell possessing no stem cell properties into contact with a factor secreted by an organ of a neonatal or juvenile mammal. The step may be a step of (1) transplanting the vascular endothelial cell possessing no stem cell properties into the organ of the neonatal or juvenile mammal, wherein the mammal is a non-human mammal, or (2) culturing the vascular endothelial cell possessing no stem cell properties in a culture system containing the factor secreted by the organ of the neonatal or juvenile mammal.