A centrifugal blower comprising a siroco fan having a plurality of blowing fans, and a scroll casing forming a spiral draft passage at an external side of the siroco fan, where a siroco fan is so mounted that spiral centers (02,03) comprising the draft passage of the scroll casing are detached from a center (01) of the siroco fan at a predetermined distance, such that a spiral curve of the scroll casing is simply changed to a non-linear curve to thereby enable to increase a flow rate even at a low revolution of the siroco fan and to reduce noise.