PURPOSE: To reduce noise at a low cost by setting a distance between a fan and a shield plate to a position where the incident wave, advancing vertically toward a shield plate, of the acoustic waves of noise generated by a fan is offset by reflection waves generated during reflection by the shield plate.

CONSTITUTION: A shield plate 9 is arranged on the front of a hood 10, i.e., the front of a fan 2. The length in the axial direction of a hood 10 and the mounting position of a shield plate 9 are selected so that a distance Ls between the shield plate 9 and the fan 2 is set to a position where an incident wave 11, advancing in the direction extending vertically to the shield plate 9, of acoustic waves of noise generated by the fan 2 is just offset by a reflection wave 12 generated through reflection by the shield plate 9. The incident wave 11 advancing toward the shield plate 9 and occupying most of acoustic waves of noise generated by the fan 2 is offset by the reflection wave 12 generated by the shield plate 9 and therefore, the maximum noise is silenced.