PURPOSE: To obtain a powerful defoaming effect by rotating defoaming vanes of specified simensions at a relatively low speed as the agitator vanes for removing foams occurring in fermentation tank or the like.

CONSTITUTION: A disc 2, as an upper plate, is fastened to a rotatingshaft 1 by supporting with a boss 5. A plurality of vane plates 3 are fitted to the lower surface of this upper plate 2 radially, and a conical form having a circular opening A in the top is formed in a lower plate 4. Suppose the area of the opening A is Sa, the effective area of injection part consisting of the product of the closest distance between the upper plate 2 and lower plate 4 in the circumferential part B and the length of the circumference, Sb, and the effective area of suction part consisting of the product of distance D between upper plate 2 and lower plate 4 on the circumference of opening hole A and the length of circumference, Sc. By setting the ratio of Sb and Sa at 1 to 2, and that of Sb and Sc at 0.3 to 1, and the rotating speed of rotating shaft 1 at 1500 rpm, the defoaming power is extremely increased.