Storage tank the liquid is stored; A liquid supply unit for keeping constant the liquid retention of the storage tank; A pilot line extending toward the storage tank from the liquid supply unit; Located in the pilot line end region checking the liquid level in the storage tank to detect the level of determining the supply of the liquid supply unit portion; And to adjust the height of the water level sensing portion pilot line end region moving up and down parts of the water level detected by the water level control unit that sets a reference liquid retention of the storage tank; the storage tank water level control device including a is introduced.