To allow one closing member to enhance the seal effect of the other closing member by improving a fitting method when closing a lid by a plurality of closing members with respect to a cylindrical opening part.

The lid closing structure comprises a lower unit provided with a throwing cylindrical part having a predetermined seal surface on an outer circumferential rib, and a bottom lid 22 having a flat columnar projecting part on the side opposite to the throw-in cylindrical part. A packing 30a having a U-shaped section is mounted on a projecting part of the bottom lid 22, and a packing cover 30b is mounted so as to cover the packing 30a. When the bottom lid 22 is closed at the throw-in cylindrical part, the projecting part of the bottom lid 22 is inserted at the entering position in an inner circumferential rib of the throw-in cylindrical part. The packing 30a is combined so as to be across the inner circumferential rib of the throw-in cylindrical part, the outer circumferential surface of the packing 30a is brought into contact with the seal surface of the outer circumferential rib, and the packing cover 30b is combined in a pressing manner with the inner circumferential rib of the throw-in cylindrical part.