indicated by arrows b providing other causes of noises and turbulent flows in the air exhausting device fig6 and 7 illustrate a preferred embodiment of the present invention to eliminate such a problem by establishing streamlined paths in the individual passageways between the guide vanes 41 referring to fig6 and 7 the air exhausting device embodying the present invention comprises a motor housing 69 consisting of a generally annular rim portion 70 an outer cylindrical portion 71 extending axially rearwardly from the inner circumference of the rim portion 70 an inner cylindrical portion 72 radially inwardly spaced apart from the outer cylindrical portion 71 and a plurality of second guide vanes 73 integrally interconnecting the outer and inner cylindrical portions 71 and 72 the outer and inner cylindrical portions 71 and 72 are arranged substantially similarly to their counterparts of the bracket 62 of the motor housing structure 61 of the embodiment of fig3 and 4 the second guide vanes 73 are essentially similar to their counterparts of the embodiment of fig3 and 4 but are herein assumed to terminate halfway of the cylindrical portions 71 and 72 the annular rim portion 70 of the housing 69 is provided with a suitable number of radial projections 74 extending radially outwardly from the outer circumference of the rim portion and has formed in its front wall depressions 75 having flat bottom faces located at the downstream end of the annular passageway 31 around the rotor 23 and which are flush with and merge into the front end faces of the radial projections 74 respectively each of the projections 74.