To reduce costs by making it possible to remove air bubbles without externally attaching a liquid cyclone or the like.

A circulating tank 2 arranged on the way of a liquid circulating line is partitioned into left and right chambers by a partition plate 4, and a liquid receiving region 6 and an air bubble removing region 7 are formed in one chamber side by side in the flowing direction of a liquid 1 while the other chamber is set to a liquid storage region 8. A plurality of fin plates 15, which have dog leg-shaped bent parts 13 formed to the end side parts thereof on the downstream side of the flowing direction of the liquid 1 and have air bubble floating notch parts 14 formed to one ends of the bent parts 13, are inclined in a left and right direction so that an up-grade is formed in a before and behind direction and the notch parts 14 are located at a high position to be laminated in a multistage fashion to form air bubble removing units 11 and these units are arranged in the air bubble removing region 7. The downward end of the bubble removing region 7 and the liquid storage region 8 are connected by a liquid transfer pipe 20 equipped with a transfer pump 19.