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