To constitute a lid receiving structure including a structure for preventing a cover plate from floating so that it may not interfere with operation at the time of entering/exiting from an opening and also to simplify its construction.

A box-like body 15 forming a fitting cavity 13 is fixed on a lower face of a receiving piece 10 of a lid receiving frame 7 exhibiting an angle. The fitting cavity 13 is opened inward in an opening 3. A rotary-type engaging protrusion 30 provided on a lid plate 5 can be fitted into the fitting cavity 13. The engaging protrusion 30 is in contact with an upper face 36 of the fitting cavity 13, thereby preventing the lid plate 5 from floating.