PURPOSE: To constantly feed mixed water of a constant concentration from a supply valve connected to multiple sub tanks by feeding and mixing a constant volume of stock solution and diluted water automatically by signals from three liquid level sensors in a main tank.

CONSTITUTION: Pipes from a stock solution feeding source 10, diluted water feeding source 17, and a compressor 20 are connected to the receiving side of a main tank 14, and a delivery pipe 26 to sub tanks 27 is connected to a delivery side. When the main tank 14 is empty, a three-way valve 22 in an air supply pipe 21 is opened by signals from liquid level sensor 23 to reduce the inner pressure of the main tank 14 to zero. At the same time, a valve 16 in a liquid feed pipe is opened to feed the stock solution by a pump 11. When a liquid level reaches the liquid level sensor 24, supply of the stock solution is stopped, the valve 19 is opened to feed diluted water and mixed with the stock solution. When the liquid level reaches the level of the liquid level sensor 25, the valve 19 is closed, the valve 22 is switched over, and the compressor 20 pressurizes the inside of the main tank 14. when the sub tank 27 is empty, the valve 28 in the delivery pipe 26 is opened by signals from the liquid level sensor 32 to feed the mixed water to the level of the liquid level sensor 31.