A centrifugal blower has a fan rotatable by a rotational drive source and a scroll casing housing the fan and having first and second discharge passages. The first and second discharge passages have radial dimensions or widths and vertical dimensions which are progressively greater toward an opening. The first and second discharge passages are surrounded by a radially outer wall, a slanted strip joined to the outer wall, and a joint skirt interconnecting an annular step on which the fan is mounted and the slanted strip.