An object of the present invention is to provide a sac-like structure enclosing hematopoietic progenitor cells and a method for preparing the sac-like structure as well as a method for efficiently preparing blood cells such as mature megakaryocytes and platelets from the sac-like structure. The present invention provides a sac-like structure enclosing hematopoietic progenitor cells, the sac-like structure being obtained by plating ES cells onto feeder cells and culturing the ES cells under suitable conditions for inducing hematopoietic progenitor cell differentiation. Moreover, the present invention provides a method for producing various blood cells, the method comprising further culturing hematopoietic progenitor cells enclosed in the sac-like structure under suitable conditions for inducing blood cell differentiation.
