To remove floating matters and foam on a liquid surface of a reaction device and a culture device or defoam and to enhance gas-liquid exchange efficiency and reaction efficiency by providing the device with a porous plate on the liquid surface at a right angle and a driving mechanism for driving the porous plate in the horizontal direction.

A wire mesh 2 being one form of the porous plate is disposed on a rotary shaft 1 over above and below the liquid surface 3. The rotary shaft 1 is rotated in such a speed enabling to remove well floating matters or foam with the driving mechanism. When the device is used as a foam removing device, the foam present in a turning radius of the wire mesh 2 is pushed with the porous plate and collected to a pushing surface of the porous plate and one portion of it is pushed out to the outside of the turning radius. The liquid surface is exposed at the back face of the porous plate and gas-liquid contact is formed and gas exchange efficiency is increased. On the liquid surface, an exchange of a surface layer liquid with a deeper part liquid is generated in accordance with the movements of the porous plate and the floating matters and/or foam, thus gas-liquid contact efficiency is further improved.