To muffle NZ noise generated in a casing in a region of the casing.

A vortex flow blower 1 as an example is equipped with the casing 2 using a casing body 21 wherein each of walls 79, 79 for enlarged parts, which are the walls in the region contacting with the enlarged parts 74a, is formed with a through hole 22 reaching a central portion of the enlarged part 74a, in contrast to a conventional example, and resonant mufflers 3 for muffling the NZ noise are installed to the casing body 21 with their short pipe portions inserted into the through holes 22. Different embodiments of the invention may be variations equipped with resonant mufflers having values of muffle frequency f different from each other in the respective through holes 22, equipped with resonant mufflers continuously adjustable of the values of muffle frequency f and equipped with resonant mufflers on a wall of the casing in a region contacting with the vicinity of an end portion on a side where a partition of the enlarged part 74a is formed.