PURPOSE: To simply lower noise such as peak sound or the like by forming a wall thick part in the peripheral part of a partition plate partitioning between a suction port and a discharge port in a cover of a casing, and by setting the thickness of the wall thick part to a value greater than that of a part other than the peripheral part of the partition plate.

CONSTITUTION: A casing surrounding a disc-like impeller 1 and defining in its outer peripheral part an annular vent passage 2, is divided axially into a casing body 3 and a cover 4. Further, the casing body 3 is formed therein axial suction and discharge ports 5, 6 which are communicated with the vent passage 2. Further, a partition plate 7 partitioning between the suction port 5 and the discharge port 6 is disposed on the inner surface of each of outer peripheral parts of the casing body 3 and the cover 4. In this arrangement, a thick wall part 4x is formed in the peripheral part of the partition wall 7 in the cover 4, the thickness of the thick wall part being set to a value which is larger than that of a part 4n other than the peripheral part of the partition plate 7. Thereby it is possible to simply reduce noise such as peak sound or the like.