A sealing structure for a casing, comprising: a case having an opened top; a cover that is attached to the top of the case to form a flow path for a fluid therein; the case and the cover having upper end surfaces and, which serve as mutual butting surfaces between the case and the cover, a center plate between the case and the cover, the center plate partitioning an inner space into a side of the case and a side of the cover; and a gasket between the case and the cover, wherein the gasket is formed in a shape conforming to the upper end surfaces (211, 311) of side peripheral walls (21, 31) of the case and the cover to surround an entire upstream chamber and an entire downstream chamber of the case, and has a circular bead portion on the entire peripheries of the upper end surfaces (211, 311), forming a sealing line sealing a space between the case and the cover, the center plate is formed so as to have a size that is within an inner peripheral side relative to the sealing line of the gasket and is stacked on the gasket, and the cover is attached to the top of the case by sandwiching the stacked gasket and center plate therebetween so as to compress the gasket with the mutual butting surfaces (211, 311) between the case and the cover, the interior of the case is partitioned into the upstream chamber and the downstream chamber by a partition wall, which extends across the opposing sides on the side peripheral wall, wherein the upper end surface of the partition wall is formed so as to have the same height as the upper end surface of the side peripheral wall, the center plate is configured to block the top of the upstream chamber of the case except having a flow path hole, wherein a supporter is formed on an inner peripheral side of the sealing line of the gasket to support the stacked center plate, wherein the supporter is a plane portion to support the center plate with a surface. a case having an opened top; a cover that is attached to the top of the case to form a flow path for a fluid therein; the case and the cover having upper end surfaces and, which serve as mutual butting surfaces between the case and the cover, a center plate between the case and the cover, the center plate partitioning an inner space into a side of the case and a side of the cover; and a gasket between the case and the cover, wherein the gasket is formed in a shape conforming to the upper end surfaces (211, 311) of side peripheral walls (21, 31) of the case and the cover to surround an entire upstream chamber and an entire downstream chamber of the case, and has a circular bead portion on the entire peripheries of the upper end surfaces (211, 311), forming a sealing line sealing a space between the case and the cover, the center plate is formed so as to have a size that is within an inner peripheral side relative to the sealing line of the gasket and is stacked on the gasket, and the cover is attached to the top of the case by sandwiching the stacked gasket and center plate therebetween so as to compress the gasket with the mutual butting surfaces (211, 311) between the case and the cover, the interior of the case is partitioned into the upstream chamber and the downstream chamber by a partition wall, which extends across the opposing sides on the side peripheral wall, wherein the upper end surface of the partition wall is formed so as to have the same height as the upper end surface of the side peripheral wall, the center plate is configured to block the top of the upstream chamber of the case except having a flow path hole, wherein a supporter is formed on an inner peripheral side of the sealing line of the gasket to support the stacked center plate, wherein the supporter is a plane portion to support the center plate with a surface.