To provide a blood processor capable of preventing (or suppressing) the generation of internal bleeding and internal bleeding marks when a hollow needle punctured to the blood vessel of a donor or a patient comes off from the blood vessel at the time of reinfusion.

A blood component sampling device (blood processor) 1 comprises a blood component sampling circuit 2 provided with a blood collection/blood reinfusion line 21 and a hollow needle assembly 29 including the hollow needle 291 freely detachably connected to one end side of the blood collection/blood reinfusion line 21, a blood pump 11 for feeding blood inside the blood collection/blood reinfusion line 21, a donor pressure sensor 18 for detecting a donor pressure which is a pressure inside the blood collection/blood reinfusion line 21 on the side of the hollow needle 291 by the blood pump 11, and a control means 55. When the blood reinfusion by feeding blood components from the side of the blood pump 11 to the side of the hollow needle by the operation of the blood pump 11, the control means 55 obtains the elevating rate of the donor pressure on the basis of the detected result of the donor pressure sensor 18, and stops the blood pump 11 when the value exceeds a threshold A.