It is intended to provide a simple method and device for removing unnecessary cells such as erythrocytes and granulocytes from a mammalian body fluid and preparing a cell fraction containing monocytes. The cell fraction containing monocytes is prepared by the combination of a treatment with a granulocyte-adsorbing material and centrifugation, a cell separating agent, or removal of unnecessary cells using a cell separating filter. The invention also provides the device for preparing the cell fraction containing monocytes, comprising a unit of treating a mammalian body fluid with the granulocyte-adsorbing material and a unit of removing fractions of unnecessary cells such as erythrocytes by a method for centrifuging the treated cell fraction, a method using the cell separating agent, or a method using the cell separating filter.
