To provide a small-sized and inexpensive apparatus for removing dirt in a water tank, easy to make operations for its installment and maintenance.

The apparatus for removing dirt in a water tank has the following construction and mechanism: a long-and-narrow-tubeform vessel with its one end left open as an opened end and the other end closed as a closed end is provided with a drain port in the proximity of the closed end, and sunk into rearing water to a predetermined position from the opened end side to project the upper portion of the vessel including the closed end and the drain port above the water surface, thus forming a space partitioned by the water surface and the inner wall surface of the upper portion of the vessel. For supplying bubbles floating toward the space, a foamed material is placed in the proximity to the closed end.