A blood purification device includes a blood circuit for extracorporeally circulating blood of a patient: a blood purifier provided on the blood circuit; a gas-liquid separator being provided on the blood circuit on the downstream side of the blood purifier on a blood flow and separating air bubbles contained in inflowing blood: and a liquid level adjustment unit capable of adjusting a liquid level height in the gas-liquid separator. The liquid level adjustment unit performs control so that the liquid level height during priming is higher than the liquid level height during treatment.