It is an object of the present invention to provide a sealing structure for a casing that can reduce the number of used gaskets and achieve further weight reduction, with the sealing function of the gaskets secured. The object is solved by including: a case 2 having an opened top; a cover 3 attached to the top of the case 2 to form a fluid flow path therein; a center plate 5 interposed between the case 2 and the cover 3 and partitions an inner space into the side of the case 2 and the side of the cover 3; and a gasket 4, which is disposed between the case 2 and the cover 3, and forming the center plate 5 so as to have a size that is within an inner peripheral side relative to a sealing line 41 of the gasket 4 and stacking this center plate on the gasket 4, and attaching the cover 3 to the top of the case 2 with the stacked gasket 4 and center plate 5 sandwiched there between to compress the gasket 4 with the mutual butting surfaces between the case 2 and the cover 3.