PURPOSE: To reduce contraflow air, increase static pressure and improve ventilating efficiency by forming arcuately the impeller side end of a cylinder section surrounding the front side outer periphery of a motor while making the configuration of the end constant.

CONSTITUTION: The impeller 7 side end of a cylinder section 10 of a tubular body case 3 is formed arcuately. As the dimensional ratio of the outer diameter D1 of cylinder section 10 to the outer diameter D2 of hub of impeller 7, i.e. D1/D2, is increased, cut-off static pressure P is increased, while the maximum blow rate Q is gradually reduced. However, the attenuation factor of blow rate does not matter with respect to the performance within 1.0W1.2 of dimensional ratio D1/D2. That is, the outer diameter D1 of cylinder 10 is preferably 1.0W1.2 times the outer diameter D2 of a hub 6.