To provide a corrosion-resistant cover lid suppressing corrosion caused by corrosive gas, a seal structure constructed adjacent thereto to enhance the mutual seal effect of the cover lids, and water treatment facilities enhanced in corrosion resistance.

The cover lid 1 has such a beam structure that beam materials are bonded to the back surface of a surface plate 10 and is constructed so as to be laid across the receiving frames P1 of the lid receiving parts P of a water tank or water channel generating corrosive gas. Main beams 11 are bonded to the back surface of the square flat plate-shaped surface plate 10 along both longitudinal side edges thereof and a bottom plate 14 is bonded to the main beams 11 so as to be at least laid over the part held between the mutual lower edges of the main beams 11 and a corrosion-resistant resin layers is provided on the undersurface of the bottom plate 14 and both end parts of the bottom plate 14 are constructed on the receiving frames P1.