entrained in the oil and having a particle diameter of at least about 0 1 mm coalesced and formed a coherent mass in the region around the inflow port 3 when the bubbles have coalesced into a large coherent mass in the manner described above the balance between the inverse force toward the inflow port 3 and the fluid force collapses so that the fluid naturally flows toward the downstream side if the fluid outflow pipe 6 is connected to a tank or other receptacle the coherent mass b of bubbles having great buoyancy easily rises of its own accord and is allowed to dissipate however as the bubble mass b bursts out into the tank or other receptable through the outflow port 8 and the fluid outflow pipe 6 it is likely to disintegrate into individual bubbles of smaller diameter due to impingement or other factors it is therefore preferable to remove the coherent mass of bubbles at the point when they coalesce into the coherent mass an air vent pipe 10 equipped with a valve 9 is connected to the center of the cylinder 1 on the lid 2 or the vortex center as shown in fig1 when the valve 9 is opened the bubbles that have coalesced into the coherent mass b at the center of the cylinder 1 near the inflow port 3 are natually discharged through the air vent pipe 10 because the pressure at the center axis of the cylinder.