An impeller of a centrifugal fan having fifty or more blades of not larger than 250 mm in outer diameter which has a casing and a multi-blade impeller rotatably supported in the casing, wherein a centrifugal force is applied on air entered into an inlet formed on the casing when the impeller is rotated, and an air of high pressure is taken out through an outlet formed on a portion of the casing. An outer peripheral surface of the impeller is inclined or curved so as to have an inlet side large diameter portion and a blade holding base side small diameter portion, or is stepped so as to have an inlet side cylindrical outer peripheral surface of large diameter and a blade holding base side cylindrical outer peripheral surface of small diameter connected to the inlet side cylindrical outer peripheral surface. The inlet side cylindrical outer peripheral surface and the blade holding base side cylindrical outer peripheral surface are substantially the same height.