To simplify a structure, to facilitate mounting work and to completely secure sealing properties even under changed pressure and impact in a container.

A gasket 16 formed of an annular body having the height larger than the sum of the depths of inserting grooves 28 is fitted to the gasket inserting grooves 28 formed on the releasing end surfaces of a case main body 12 and a cover body 14 and formed into the approximately same end surface shapes. The center part in the height of the gasket is formed of a gentle expansion part 16a and a projecting part 16b having an approximately rectangular section into the thickness a little larger than the widths of the gasket inserting grooves 28, and the ends 16c are formed on bent surfaces 16d whose side surfaces on the side opposite to the side surface on the bent side are thinned toward the tip. When the gasket 16 is inserted into the inserting grooves 28, the bent surface 16d are guided to the bottom surface of the gasket inserting grooves 28, automatically bent to the side opposite to the bent surface 16a and bonded, and therefore, a clearance between the case main body 12 and the cover body 14 is completely sealed, and sealing performance is not lost in relation to impact from the outside and a pressure change in a container.