A centrifugal blower provided with a spiral shaped scroll chamber where a bottom part of the scroll chamber gradually expands downward in the axial direction of the fan well along with expansion of the spiral and where a flow area gradually expands toward an air outlet from a spiral start part of the casing, the centrifugal blower having an initial slant angle θ0 at the spiral start part of the bottom part of the scroll chamber of a range of angle of 5.2° to 27.5° or setting a backflow prevention rib at the fan outlet.