PURPOSE: To enable the taking-out of sample after centrifugal separation to be easily performed by providing a sample mounting hole to a rotor body and disposing an extraction hole of taking out the sample in the bottom part of this hole by way of a passage opening-closing mechanism.

CONSTITUTION: A valve switching screw 90 is operated to close the passage of a sample extraction hole 8 by a passage switching valve 9. A density gradient is beforehand made in a sample filling hole 3 which is a sample liquid well part, and a sample is superposed thereon, thence a cap is locked to a rotor body. The sample is replenished and filled in the filling hole 3 through the hole 10 provided to the cap 5, after which a sealing screw 11 is removed, the hole 10 is opened, the valve switching screw 90 is turned to open the passage switching valve 9 and the extracted liquid is allowed to drop from the sample extraction hole 8. This is segmented and fractionated.