To especially increase corrosive gas resistance and to achieve highly accurate functions for a long time without disturbing the response property of a sensor element by providing a film containing a nonionic water soluble resin and a slightly soluble basic metal salt.

A case 2 that protects a humidity sensor 1 and is made of plastic such as polypropylene is used, and the lead wire 6 of the sensor 1 is extended from a rear end. A window 3 is formed opposite to a humidity- sensitive part in the case 2, and the sensor 1 is sealed in the other parts. Then, a filter film 4 containing a water soluble resin and a basic metal salt is mounted along the peripheral edge of the window 3. In this case, the filter film 4 is formed along an inner surface from the opposing inner surface of the case 2. For example, the water soluble resin such as polyvinyl alcohol is dissolved into a solvent, and a dispersion object where a basic metal salt such as carbonate and hydroxide is added is dipped into a material such as a non-woven cloth for drying, thus creasing the filter film 4.