To provide a method for preventing a level sensor from malfunction by removing a reaction product with a simple operation if a liquid level sensor tube is clogged, or regularly.

In an automatic cleaning device comprising a treatment tank 3 to which a liquid chemical 7 is supplied, tubes 4, 5 mounted inside the tank 3, a liquid level sensor 6 which sends a gas into the tubes 4, 5 at a constant pressure and detects a differential pressure between a pressure in the chemical 7 and the atmospheric pressure to output a liquid level signal in the tank 3 and a liquid chemical supply section 15 which supplies and stops the liquid chemical to the tank 3 in accordance with the liquid level signal outputted by the level sensor 6, a tube cleaning means 8 for pressurizing a fluid into the tubes 4, 5 is provided.