In a blood purification device containing a blood purifier (1), a blood circuit (2), a blood pump (3), and a dialysate line (4) having a fresh dialysate supply line (41) and a used dialysate discharge line (42), a pair of plunger pumps (51, 52) are disposed in the dialysate line (4). The pair of plunger pumps (51, 52) are synchronized so that delivery of a fresh dialysate from one plunger pump (51) and suction of a used dialysate into the other plunger pump (52) simultaneously occur and the stroke of at least one of the pair of plunger pumps (51, 52) is made variable.
