PURPOSE: To improve the efficiency and prevent the stall of circulation flow by forming at least the section of the air passage of a casing into two circular shape and permitting the efficient conversion between the energy of speed and the energy of pressure.

CONSTITUTION: A circulation flow 11 which gets into the inside of an impeller 3 is accelerated by being given an energy of speed by a centrifugal force due to the revolution of the impeller 3, and then discharged into an air passage 10 of a casing 6 from the impeller 3, and converted into pressure. As the outside radius γ1 of the casing 6 is made small, pressure conversion is performed, eliminating the loss due to directional conversion. As the inside radius γ2 of the casing 6 is made small, pressure conversion is performed, eliminating the loss due to directional conversion. As the inside radius γ2 of the casing 6 is made large. large, the circulation flow 11 returns on the impeller 3 side smoothly, and so stall state can be eliminated, and the circulation flow 11 in high pressure in the air passage 10 does not flow backward into the intake port 8 side in low pressure, and the efficiency is improved sharply