To greatly reduce the vibration and noise to be propagated from a fan casing to an external cylinder by fitting the noise absorbing material to an inner surface of the external cylinder, providing the fan casing on the inner circumferential side of the noise absorbing material, and forming a recessed groove to provide an air layer on the outer circumferential surface of the noise absorbing material at a maximum diameter part of the fan casing.

A mixed flow blower to be used in a ventilation unit to ventilate the air in a building, etc., sucks the suck-in flow 13 from a suction port 10 into an external cylinder 7, and discharges the flow through the mixed flow impeller 2 and a return guide vane 5 by driving a mixed flow impeller 2 having a plurality of blades 1 by a motor 6. The noise absorbing material 9 is provided on the inner circumferential surface of the external cylinder 7, projections 16 are provided on the outer circumferential surface of the noise absorbing material 9 to form an air layer 17. The noise absorbing material 9 is filled between the fan casings 3, 4 and the, external cylinder 7, and an air layer 17a is provided through the noise absorbing material 9 at a maximum diameter part of the fan casings 3, 4. The air layer 17a makes it difficult for the vibration to be propagated to the external cylinder 7 to obtain the sufficient noise reducing effect.