The present invention is a water tank (1) with a float which is separate from a lower stage water tank and is provided with a water tank part (2) having an opening part (4) that can be immersed with respect to a water surface of a lower stage water tank (5) having water therein, and a float part (3) provided continuous with the water tank part (2) directly below the opening part. The gravitational force when the water tank part (2) is filled with water and the buoyancy of the float part (3) are adjusted such that the water tank can stand independently and movably in any position in the lower stage water tank (5).
