To prevent a foreign article from adhering to a pressure sensitive part and maintain the function of the pressure sensitive part long, by covering the pressure sensitive part set at an upper part of a container opened at a lower part thereof with a liquid of a small specific gravity.

A pressure sensitive part 110 is set at an upper part of a container. A decoding cable 140 consisting of a vent pipe, a power source plug.signal line is fitted so that the pressure sensitive part 110 balances with an atmospheric pressure. A neck part 120 is formed at the container to narrow a central part, thereby preventing mud or foreign articles of a large specific gravity in the down water from being in touch with the pressure sensitive part 110. When the container is formed slim and the neck part 120 is further narrowed, the mud and foreign articles are more difficult to be in touch with the pressure sensitive part 110. Furthermore, if a heater is set above the neck part 120 in the container to warm the water, a specific gravity of the water is reduced, so that the water of the smaller specific gravity rises along a side face of the container and stay in the periphery of the pressure sensitive part 110. Accordingly, the mud and foreign articles of the large specific gravity in the water do not adhere to the pressure sensitive part 110.