PURPOSE: To aim at reducing resonating sound, voltex sound, etc. in the casing of a centrifugal blower, by forming the casing with the use of a composite vibration damping steel plate structure or the like, which is formed by clamping a viscoelastic polymer material such as, for example, rubber having a sound absorbing characteristic, between steel plates.

CONSTITUTION: A viscoelastic polymer material 6 such as, for example, rubber, having a sound absorbing characteristic is applied on one surface of a seel plate 5, and further, another polymer material is clamped between steel plates 5 from both sides thereof. Thus, composite vibration damping steel plate structures 3, 4 are prepared. Further, the steel plate structure 3 is formed in a sprial shape with the viscoelastic polymer material 6 facing outward to form a spiral plate 9. Or the steel plate structure 4 is formed into a spiral shape to form the sriral plate 9. Further, a side plate 7 formed of the steel plate structure 3 or 4 is used to form a suction port 8. Further, with the use of the sprial plate 9 and the side plate 7 a blow-out port 10 is formed. Since the blower casing is formed of a vibration damping steel plate structure, resonating sound and voltex sound in the casing may be reduced.