A blood purification device (1; 1a; 1b) comprising: a chamber that is provided on a blood circuit for extracorporeally circulating patient's blood and configured to introduce purified plasma obtained by purifying plasma separated by a plasma separator provided on the blood circuit, or a replenishing liquid for replenishing the plasma separated by the plasma separator, into the blood circuit; a liquid feed line capable of sending the purified plasma or the replenishing liquid to the chamber; an air introduction unit capable of introducing air into the liquid feed line; a liquid level adjustment unit configured to be capable of adjusting a liquid level height in the chamber and configured to be capable of increasing the liquid level height in the chamber by discharging air from the inside of the chamber; a control unit that is configured to, at an end of blood purification treatment, perform a liquid recovery process for sending the purified plasma or the replenishing liquid to the chamber via the liquid feed line while introducing air into the liquid feed line by the air introduction unit and maintaining the liquid level height in the chamber at a predetermined liquid level height by the liquid level adjustment unit; a first liquid level detection sensor being provided at a first height position of the chamber and being capable of detecting whether or not a gas is present at the first height position in the chamber, wherein the control unit causes the liquid level adjustment unit to perform an operation of discharging air from the inside of the chamber when a gas is detected by the first liquid level detection sensor; and a second liquid level detection sensor being provided at a second height position of the chamber, which is higher than the first height position, and being capable of detecting whether or not a gas is present at the second height position in the chamber, wherein the control unit starts discharge of air from the inside of the chamber by the liquid level adjustment unit when a gas is detected by the first and second liquid level detection sensors, and stops the discharge of air by the liquid level adjustment unit when a gas is not detected by the first and second liquid level detection sensors. a chamber that is provided on a blood circuit for extracorporeally circulating patient's blood and configured to introduce purified plasma obtained by purifying plasma separated by a plasma separator provided on the blood circuit, or a replenishing liquid for replenishing the plasma separated by the plasma separator, into the blood circuit; a liquid feed line capable of sending the purified plasma or the replenishing liquid to the chamber; an air introduction unit capable of introducing air into the liquid feed line; a liquid level adjustment unit configured to be capable of adjusting a liquid level height in the chamber and configured to be capable of increasing the liquid level height in the chamber by discharging air from the inside of the chamber; a control unit that is configured to, at an end of blood purification treatment, perform a liquid recovery process for sending the purified plasma or the replenishing liquid to the chamber via the liquid feed line while introducing air into the liquid feed line by the air introduction unit and maintaining the liquid level height in the chamber at a predetermined liquid level height by the liquid level adjustment unit; a first liquid level detection sensor being provided at a first height position of the chamber and being capable of detecting whether or not a gas is present at the first height position in the chamber, wherein the control unit causes the liquid level adjustment unit to perform an operation of discharging air from the inside of the chamber when a gas is detected by the first liquid level detection sensor; and a second liquid level detection sensor being provided at a second height position of the chamber, which is higher than the first height position, and being capable of detecting whether or not a gas is present at the second height position in the chamber, wherein the control unit starts discharge of air from the inside of the chamber by the liquid level adjustment unit when a gas is detected by the first and second liquid level detection sensors, and stops the discharge of air by the liquid level adjustment unit when a gas is not detected by the first and second liquid level detection sensors.