Priming and fluid replacement of a blood purification device is suitably performed by using a pump having a flow rate and a flow rate accuracy suitable for priming. The blood purification device includes: a blood purification unit (20) having a blood purification membrane (30) a blood circuit (10) that feeds blood in a blood collection part (21) to a primary side of the blood purification membrane (30) and returns the blood from the primary side to a blood return part (23) by using a blood pump (40) a dialysate supply circuit (91) that supplies the dialysate to a secondary side of the blood purification membrane (30) a dialysate discharge circuit (92) that discharges the dialysate from the secondary side and a fluid replacement/priming circuit. While the blood collection part (21) and the blood return part (23) are connected to each other to form a loop shape of the blood circuit (10), the dialysate in the dialysate supply circuit (91) is supplied to the primary side through the blood purification membrane (30) by using a fluid replacement pump (170), and the dialysate in the blood circuit (10) including the primary side is discharged to the dialysate discharge circuit (92) without passing through the blood purification membrane (30) by using a fluid replacement pump (170).
