Abstract: This invention relates to an apparatus used in a centrifugal separating step for sample preparation in the field of clinical chemistry. Especially, the invention relates to a separating apparatus used for isolating a serum fraction from a freshly isolated blood sample without contamination with a clot fraction. A serum separating apparatus according to the present invention is characterized by comprising an elastic body in the form of a cylindrical tube and a ball which can fitted to an opening of the elastic body and by positioning at a boundary of said fractions after the centrifugation.