To carry out confirmation of an operating condition and inspection of a stop part without dismounting a cover by manufacturing the cover with a transparent material, in a device formed in such a constitution that a valve hole opened/closed by a valve element which is lifted together with a float in a valve casing is covered with a cover mounted on the valve casing.

In an initial condition in which piping of a water circulating pipe 1 is completed, a float 3 and a valve element 5 fall down on a valve opening position, and is held. A valve hole 6 is opened, and air in the water circulating pipe 1 is rapidly delivered from a valve casing 2 outward when water is filled to the water circulating pipe 1. A float guide 4 is floated by the float 3 and the valve element 5 in association with flow-in of water into the valve box 2, the valve hole 6 is closed, and filling of water is stopped. In this case, a cover 9 is made of a transparent material such as acrylic reinforced plastic, and is mounted on the valve casing 2 using a plurality of stay 8A and a butterfly bolt 8B so as to cover the valve hole 6. It is thus possible to carry out inspection whether water is leaked from a water stop part or not while being transparent to the cover 9.