A main air bubble separation apparatus which uses a swirling flow of oil introduced from a main tank separates the oil into a first oil with air bubbles removed therefrom, and a second oil containing air bubbles; a sub-air bubble separation apparatus is provided in which the second oil is introduced and then separated into a third oil with air bubbles removed therefrom and a fourth oil containing air bubbles; and a sub-tank into which the fourth oil is introduced, and in which the air bubbles contained in the fourth oil are captured and combined with each other so as to be removed from the fourth oil, so that a fifth oil is formed, and is configured such that the first, third and fifth oils are returned to the main tank.