To provide an air lift pump for installing water treatment system capable of preventing a change of quantity of the water to be discharged while reducing the fluctuation of a pump head with increase and decrease of quantity of the water with the simple structure.

This air lift pump for installing water treatment system is provided in a waste-water system provided with plural storage and treatment chambers so as to transfer the treated water from one storage and treatment chamber to other storage and treatment chamber. This air lift pump for installing water treatment system is provided with a lifting pipe for lifting the stored treated water with the air for transference, and an air feeding pipe provided at a lower part of the lifting pipe so as to discharge the air. The lifting pipe is provided with a lifting pipe height adjusting mechanism for increasing and decreasing the height of the lifting pipe provided in one storage and treatment chamber as a transference source in response to increase and decrease of water level of that storage and treatment chamber.