To make the water level in a protective container to smoothly rise, following the rise of the water level in a tank.

An attaching recessed section 25 is formed on a lid body 8 put on a water tank 2 and a protective tube 27, having a communicating port 2 formed through its lower end face, is provided on the bottom of the section 25. Float switches 31 and 32 are positioned in the tube 27, and an air vent hole 40 is formed through the upper end section of the peripheral wall of a tube 28. When fresh water is supplied to the tank 2, the water level in the tank 2 rises and, at the same time, the water flows into the tube 28. When the water flows in the tube 28, the water level in the tube 28 rises smoothly as the air above the water surface in the tube 28 goes out through the air vent hole 40. Therefore, the water level in the tank 2 can be detected precisely, because the water level in the protective tube 28 in which the float switches 31 and 32 are positioned rises accurately following the water level in the tank 2.