Microbubbles detached from a blood circuit and a blood purification unit are discharged with the use of a backflow generated at the instant that a roller of a blood pump releases a squeezable tube. In a normal rotation step, a region filled with a priming solution after a priming step is closed by a closing unit, and a rotor of a blood pump is rotated normally until a roller of the blood pump releases a squeezable tube to generate a backflow. After the backflow is generated at the release of the squeezable tube by the roller of the blood pump, bubbles are moved by reversely rotating the rotor while disabling the closing by the closing unit. Thus, the bubbles are discharged through a discharge unit.
