PURPOSE: To prevent the exfoliation or the mixing loss of a flow in the vicinity of the intake of an impeller by providing a ring-shaped suction casing so that a leaking flow from a labyrinth between the side plate and the casing of the impeller with side plates is sucked in just in front of an intake of a vane of the impeller and blown out along the main flow direction.

CONSTITUTION: In a centrifugal compressor, an impeller 1 is attached to a rotating shaft, and fluid is inhaled from a return channel 3 and, after passing through the impeller and pressurized, introduced to the next stage through a diffuser 2. A labyrinth 6 is provided between a side plate 1a and the casing 10 of the impeller 1, and a part of the flow from the impeller 1 returns to the inhalant side as a leaking flow 9 and flows in the impeller again mixed with the main flow 8. When, the labyrinth 6 is made in the radial direction and the axial length of the impeller side plate 1a is made short to provide a space 11 for diffusion of leaked flow from the labyrinth 6. And a ring-shaped suction casing 7 is provided extending just to the front of vane intake of the impeller 1. Thereby leaked flow from the space 11 can be blown along the main flow at the front of the intake of the impeller.