It is an object of the present invention to provide a method for efficiently preparing blood cells, such as mature megakaryocytes and platelets, from iPS cells in an in vitro culture system. The present invention provides a sac-like structure enclosing hematopoietic progenitor cells, which is obtained by inoculating iPS cells onto feeder cells and then culturing the iPS cells under conditions suitable for inducing the differentiation of hematopoietic progenitor cells. Moreover, the present invention also provides a method for producing various types of blood cells, which comprises culturing hematopoietic progenitor cells enclosed in the sac-like structure under conditions suitable for inducing the differentiation of blood cells. Furthermore, the present invention also provides a method for producing various types of blood cells, particularly megakaryocytes and platelets, without involving the sac-like structure.