PURPOSE: To enable the pumping-up of water even stagnating on tank bottom and ensure the easy mounting and dismantling of a pump by forming a pumping- out part with a lower upper wall on the part of a water tank and enabling the installation of a pumping part immersed in water at said pumping-out part.

CONSTITUTION: A water tank T for city water and the like has a pumping-out part 1 formed with a part of its upper wall 2 at a lower level but slightly above the pumping part 8 of a water lifting pump P, and an opening 3 is provided on said upper wall 2. One end of a sheathing pipe 4 is connected to the inside of the pumping part 1 of said opening 3. Also, the other end of the sheathing pipe 4 is open to the bottom of the water tank T and the opening is fitted with a valve 11 opened and closed by an operation part 12 outside of the water tank T. And the pumping part 8 is perpendicularly set within the sheathing pipe 4 and the pump P is watertightly mounted to the opening 3. The mounting and dismantling thereof are carried out with the valve 11 closed.