A casing of a centrifugal fan includes a suction port forming wall portion, an opposed wall portion opposed to the suction port forming wall portion, and a sidewall portion connecting the suction port forming wall portion and the opposed wall portion. A tongue portion is provided at the sidewall portion. The tongue portion projects into an air passage in the vicinity of a discharge port to form a narrowed air passage portion within the air passage. A leading end surface of the tongue portion is shaped such that the width of the leading end surface increases from the suction port forming wall portion, where the suction port is formed, to the opposed wall portion. The tongue portion is shaped such that the projecting length of the tongue portion into the air passage continuously decreases from the suction port forming wall portion to the opposed wall portion.