A blood purification device comprising: a blood purification unit including a blood purification membrane; a blood circuit that feeds blood in a blood collection part to a primary side of the blood purification membrane of the blood purification unit and returns the blood from the primary side of the blood purification membrane of the blood purification unit to a blood return part, by using a blood pump; a dialysate supply circuit that supplies dialysate to a secondary side of the blood purification membrane of the blood purification unit; a dialysate discharge circuit that discharges the dialysate from the secondary side of the blood purification membrane of the blood purification unit; and a fluid replacement/priming circuit configured to perform a fluid replacement process in which the dialysate in the dialysate supply circuit is supplied to the blood circuit by using a fluid replacement pump and a priming process in which while the blood collection part and the blood return part are connected to each other to form a loop shape of the blood circuit, the dialysate in the dialysate supply circuit is caused to flow out to the primary side through the blood purification membrane and the dialysate in the blood circuit including the primary side is discharged to the dialysate discharge circuit by using the fluid replacement pump, wherein the fluid replacement/priming circuit is connected to the blood circuit, and is configured to be selectively connectable to the dialysate supply circuit and the dialysate discharge circuit, and the fluid replacement/priming circuit includes a bypass circuit connecting the dialysate supply circuit to the dialysate discharge circuit, and a connection circuit connecting the blood circuit to an intermediate portion of the bypass circuit, the bypass circuit includes a first flow rate adjusting unit disposed between a connection point to the connection circuit and a connection point to the dialysate discharge circuit and configured to be freely opened and closed, and a second flow rate adjusting unit disposed between a connection point to the connection circuit and a connection point to the dialysate supply circuit and configured to be freely opened and closed, characterized in that the fluid replacement pump is rotatable forwardly and reversely and is disposed in the connection circuit. a blood purification unit including a blood purification membrane; a blood circuit that feeds blood in a blood collection part to a primary side of the blood purification membrane of the blood purification unit and returns the blood from the primary side of the blood purification membrane of the blood purification unit to a blood return part, by using a blood pump; a dialysate supply circuit that supplies dialysate to a secondary side of the blood purification membrane of the blood purification unit; a dialysate discharge circuit that discharges the dialysate from the secondary side of the blood purification membrane of the blood purification unit; and a fluid replacement/priming circuit configured to perform a fluid replacement process in which the dialysate in the dialysate supply circuit is supplied to the blood circuit by using a fluid replacement pump and a priming process in which while the blood collection part and the blood return part are connected to each other to form a loop shape of the blood circuit, the dialysate in the dialysate supply circuit is caused to flow out to the primary side through the blood purification membrane and the dialysate in the blood circuit including the primary side is discharged to the dialysate discharge circuit by using the fluid replacement pump, wherein the fluid replacement/priming circuit is connected to the blood circuit, and is configured to be selectively connectable to the dialysate supply circuit and the dialysate discharge circuit, and the fluid replacement/priming circuit includes a bypass circuit connecting the dialysate supply circuit to the dialysate discharge circuit, and a connection circuit connecting the blood circuit to an intermediate portion of the bypass circuit, the bypass circuit includes a first flow rate adjusting unit disposed between a connection point to the connection circuit and a connection point to the dialysate discharge circuit and configured to be freely opened and closed, and a second flow rate adjusting unit disposed between a connection point to the connection circuit and a connection point to the dialysate supply circuit and configured to be freely opened and closed, characterized in that the fluid replacement pump is rotatable forwardly and reversely and is disposed in the connection circuit.