A base fixed to a liquid-containing tank and a float disposed above the base are liquid-tightly secured to lower and upper ends, respectively, of a flexible member comprising a bellows cylinder. The base, the float and the flexible member define a liquid reservoir. The float rises when there is a large quantity of liquid in the reservoir and falls when there is a small quantity of liquid in the reservoir, so that a damming plate provided on the float is moved above and below the surface of the liquid in the tank. When the damming plate is moved below the liquid surface, the liquid on the surface flows into the reservoir through a liquid inlet formed in the float, and the liquid that collects in the reservoir is discharged to the exterior of the tank by a liquid discharge pipe. As the float rises, the damming plate is lifted above the surface of the liquid in the tank, thereby cutting off the flow of surface liquid into the reservoir. This operation is performed repeatedly to remove the surface liquid from the tank.