An amniotic mesenchymal stem cell mass having a high proliferation ability and a high differentiation ability can be produced by a method comprising the steps of: (A) collecting a cell mass of a mesenchymal cell from the amnion of a mammal； (B) subjecting the cell mass of the mesenchymal cell to flow cytometry to thereby sort cells, wherein a specific gate is provided in a scattergram produced in the flow cytometry； (C) and subjecting the sorted cells to subculture. Alternatively, the amniotic mesenchymal stem cell mass having a high proliferation ability and a high differentiation ability can also be produced by a method comprising the steps of: (D) collecting a cell mass of a mesenchymal cell from the amnion of a mammal； (E) subjecting the collected cell mass to initial culture for 2 to 3 days； (F) repeating the subculture of the resultant cell mass three to four times at a low cell concentration； and (G) maintaining the culture of the cells in the same culture dish until the cells become confluent when a fusiform colony of the cells is formed in the subculture.
