A defoaming device includes a) a defoaming pipe inside which a high-speed gas flow passes, b) a gas supply pipe that supplies the high-speed gas flow to the defoaming pipe, and c) a suction pipe that is connected at one end to the defoaming pipe and at the other end to a lid part or upper mirror part of a treatment tank and through which foam is suctioned from the treatment tank, wherein an ejector action of the high-speed gas flow that passes through the defoaming pipe suctions the foam from the treatment tank through the suction pipe, and a shearing action of the high-speed gas flow breaks down the suctioned foam. The defoaming device is made to efficiently eliminate foam generated at an upper part of a treated liquid in the treatment tank.