Disclosed herein is a water level controlling device for intercepting the supply of water to the water reservoir of a water purification system. The water level controlling device includes a water reservoir having a plurality of diaphragms which are provided on the lower surface of the water reservoir in such a way as to move up and down. A flow body is mounted to the lower surface of the water reservoir and defines a water circulation path, with a plurality of control parts provided in the circulation path to block the flow of water by the corresponding diaphragms. A plurality of floaters moves up and down in the water reservoir according to the water level. An actuating means moves each of the diaphragms to the corresponding control part as each of the floaters rises, thus intercepting the flow of water passing through the control part.