To increase safety measures by mounting a lid device over an opening in a buried public utility conduit located below an existing manhole lid, the lid device comprising a lid body to which a cylinder lock is attached and a mounting frame.

At an opening where an outer receiving frame provided outside a buried public utility conduit is provided with an existing manhole lid, the lid device comprises a mounting frame whose outer form is shaped to match the shape of the opening and which is provided with an entrance/exit part at its inner end; and a lid body for opening and closing the entrance/exit part of the mounting frame. The lid body is movably mounted on the entrance/exit part. The entrance/exit part of the mounting frame secured to the interior wall surface of the opening by fasteners is covered with the lid boy greater than the entrance/exit part, in such a manner that the lid body has an overlap at its peripheral edge. A plurality of locks for locking to the underside of the mounting frame are provided on the peripheral edge of the lower surface of the lid body. The locks and an actuating part existing at an operating part provided on the lid body are operatively associated with each other by an associating part and the lid body is movably mounted.