PURPOSE: To detect a boundary surface stably and exactly in case when an anticoagulant agent is mixed, by measuring each impedance, and detecting a boundary surface between blood plasma and blood corpuscles.

CONSTITUTION: An electrode 11 which has been installed to a holder 10 and has exposed only its tip from the insulating covering, an electrode inspecting nozzle 12, and a blood plasma suction nozzle 13 are made to go down together with the holder 10, in a test tube 16 for blood plasma 14 and blood corpuscles, which have been centrifugally separated. Subsequently, when it is detected by detection of impedance of the blood plasma by a detector 17 that the electrode 11 has reached the blood plasma, descent of the holder 10 is stopped, and the blood plasma is sucked and collected by a prescribed quantity by the nozzle 13. Subsequently, when the holder 10 is further made to go down, it is detected that the electrode 11 has reached the blood corpuscle 15, in the same way by a variation of impedance detected by the detector 17, and the blood corpuscles of a prescribed quantity are collected by the electrode 12. In this way, a boundary surface between the blood plasma and the blood corpuscles is detected stably and exactly even in case when an anticoagulant agent is mixed, by utilizing the impedance without utilizing DC resistance of the blood plasma and the blood corpuscles, whose difference is small.