A purifying device comprising a purifying tank (1) having an ultraviolet radiating tube (2) installed within the middle thereof and a submersible pump (3) mounted in the upper region thereof. The purifying tank is held 5 to 100 cm below the water level by floats (7a). A partition (6) divides the region adjacent to the water level into upper and lower levels, wherein only the water on the upper side of the partition (6) is sucked in by the submersible pump (3) for purification in the purifying tank (1), whereby water-bloom, algae or the like floating on water is rapidly collected and decomposed for removal.