To eliminate foreign bodies such as cockroaches or the like from entering into a gap, and improve precision of water-level detecting and working efficiency.

A detecting frame 3 having a specified gap 1 and a detecting means 4 is provided. A moving body 5 moving in the gap 1 and having a detected means 4a is provided. The moving body 5 is connected with a water- level sensor 2 moving vertically in a hollow washing tank. In the case that a water level in the washing tank is lower than a full level, a part of the moving body 5 is positioned in the gap and the detected means 4a is positioned outside the gap. When the water level reaches the full level, the detected means 4a moves in a position opposite to the detecting means 4, and water supplying is stopped.