To mechanically destroy foam which is formed and moves upward in a reaction process, with an effect that forms the foam into thin films and a centrifugal effect on the foam by placing in a reaction vessel, a disk provided with plural curved blades and plural holes that are formed in a flat surface of the disk through the disk plate and used for allowing the foam to move upward, and rotating the disk.

In this device, a disk 21 is provided with four to eight curved blades 22 arranged radially from the center of the disk so that a flat surface 24 of the disk 21 is divided into equal sections by the blades 22, wherein the front end of each of the blades 22 is formed into a perpendicular shape to the disk 21 or a shape inclined to the outside in the direction toward its upper part from its lower part. Also, long and narrow sector-shaped holes 23 for allowing foam to move upward are formed in the flat surface 24 of the disk 21 through the disk plate, near the center of the disk 21 in the space between every adjacent two of the blades 22 to construct the objective defoaming device 5. This device 5 is rotated concurrently with the rotation of agitating vanes 2 with which an agitation rotary shaft 4 is fitted in its lower part to mechanically destroy the foam. This device 5 shows strong defoaming power even in a vacuum dehydration process in which a large amount of foam is formed or even at the time of applying the device 5 for various liquid reactants having different viscosity values over a wide range.