It is possible to keep a diameter of an impeller and a cross sectional area of an air channel as large as possible while reducing a size of a centrifugal fan in a direction perpendicular to an axis. Thereby, it is possible to acquire characteristics of a high air volume and a high static pressure without lowering the efficiency. In the centrifugal fan, when the radial gap of the cross section in an air channel 10b is gradually enlarged from a clapper portion 10c toward an outlet 10d except for a specific region 10h, the cross inner sectional area of an air channel 10b is gradually enlarged from the clapper portion 10c toward the outlet 10d.