To collect hematopoietic stem cells and to easily decrease blood cell components except for the hematopoietic stem cells to leave the plasma protein component by introducing a suspension liquid of hematopoietic stem cells into a sampling device for hematopoietic stem cells having a specified structure and then treating the liquid under specified conditions.

A collecting apparatus 7 for hematopoietic stem cells is prepared by packing a container 3 having an inlet 1 and outlet 2 for a suspension liquid of hematopoietic stem cells with a fractionating layer 4 as a main component. This layer 4 consists of one of porous material, fiber aggregates and particle aggregates and has 1 to 60μm average pore diameter. A suspension liquid of hematopoietic stem cells is introduced into the collecting apparatus 7 and the liquid is made to substantially flow upward in a gravity field. A plasma which is first drained from the collecting apparatus 7 for hematopoietic stem cells is collected. Then red blood cells and blood platelets are drained and a cell aggregate containing hematopoietic stem cells is collected.