A chamber has a first opening, a second opening, and a third opening. The second opening is higher in level than the first and third openings, and is located at the highest position of a reservoir. Air bubbles are easily collected around the second opening, higher in level than the third opening, due to buoyancy. Moreover, the chamber includes an upper slope on an upper inner wall thereof such that a sectional area of the chamber becomes smaller toward the highest position of the reservoir. The upper slope causes the air bubbles not to stagnate but to be guided to the second opening along the upper slope. This makes easy discharge of the air bubbles from the chamber.