PURPOSE: To accurately and surely adjust the level of a liquid in a water tank with a simple constitution by installing a siphon tube to the tank for transferring the liquid to the other side and automatically discharging the liquid from the tank when the level becomes high or supplying the liquid to the tank when the liquid level becomes lower.

CONSTITUTION: A siphon tube section 20 formed in an inverted U-shape having openings, a liquid supply port 21 formed in its upper end section, and control valves 22 and 23 formed on both sides of the port 21 is installed to a water tank 10 so that one opening of the tube 20 can be set at a level lower than the highest water level in the tank 10 and a water sealing tube section 30 having a U-shape is connected to the other end section of the tube 20. Then a water level adjusting tube section 40 having an inverted U-shape is connected to the tube section 30 so that the level of the internal surface part 40c at the bent section of the tube section 40 can become the same as the highest water level in the tank 10. In addition, an air port 41 is connected to the upper end section of the tube section 40.