Abstract: A centrifugal fan having a sound reduction effect for noises in a wide frequency range is provided. The centrifugal fan comprises a motor to which an impeller is connected, a casing which surrounds the impeller and which has a bell-mouth-like suction port, and a bell-mouth-like orifice which has a suction port being concentric with the suction port of the casing and having a diameter as large as that of the suction port of the casing, in an outer shell, providing a plurality of protrusions in a resonant space surrounded by the orifice, the casing, and the outer shell, and providing depressions which are formed so as to be interposed between the protrusions.