Abstract: Deaerator for the treatment of oil returning from an oil circulating lubrication system before the oil returns into the oil tank (1), said deareator including an inlet container (2) acting as an air lock, in which the oil level is higher than the oil level in the oil tank (1) and into which the oil to be purified flows via a return pipe (3); a separating container (4) placed above the inlet container, with a rising pipe (5) leading from the inlet container into the separating container, an outlet pipe (6) for passing the oil from the separating container (4) into the oil tank (1), and a suction connection (7) in the upper part of the separating container (4) for the application of negative pressure in the separating container and for the removal of the separated air from the separating container.