Abstract: A blood separator having: a blood bag pressurizing section for retaining a centrifuged blood bag and pressurizing the same at a predetermined pressure; a retention/measurement section for retaining at least two separation bags connecting with the blood bag via tubes and capable of receiving separated blood components, and measuring the weight of the separation bags; limiting devices provided at intermediate portions of the tubes between the blood bag and the separation bags to limit the flows of liquids in the tubes; a detector for detecting the level of an interface between components of blood in the blood bag retained in the blood bag pressurizing section; a moving device for moving the detector relative to the blood bag; and a controller for controlling the limiting devices while receiving, as an item of control information, detection information from the detector and/or measurement information from the retention/measurement section.