The present invention provides an automatic priming method by which priming is conducted while keeping, in the automatic priming, an arterial-side chamber in a normal position as in a case of dialysis treatment, and in which forward and backward rotations of a blood pump are employed, and a speed of rotation, in particular, backward rotation is adjusted to be low so that air or the like is reliably prevented from entering a dialyzer. The priming method is characterized by including, in a blood purification circuit in which priming is conducted by supplying physiological saline into a circulation circuit formed by connecting an arterial blood circuit (L1) with a venous blood circuit (L2), (1) an early injection step of filling the venous blood circuit with a priming solution (physiological saline), (2) an overflow step of discharging the priming solution for cleaning the arterial blood circuit, (3) a reverse direction re-circulation step of conducting a treatment in a direction opposite to the overflow step, which may be followed by a forward direction re-circulation step, and (4) a forward direction re-circulation step.