A blood purification device for purifying blood drawn from a body and then returning the blood into the body, including: a pump provided midway along a fluid path through which blood or a dialysis fluid flows; a valve that is provided midway along the fluid path and closes or opens a part of the fluid path; and a control unit that causes a blood purification process of passing the blood or the dialysis fluid through the fluid path for blood purification, a priming process of driving the pump and the valve to supply a priming fluid to the fluid path before the blood purification processing, and a pressure reduction process of reducing pressure in a gap in the fluid path to a negative pressure state before supply of the priming fluid.