A centrifugal blower includes a centrifugal fan; an electric motor that rotationally drives the centrifugal fan; a fan case that has an air inlet and an air outlet and that internally accommodates the centrifugal fan; and an electric motor case (8) that has a cylinder (6) accommodating the electric motor and a flange portion (7) extending radially outward from the cylinder (6) and that is fitted to the fan case. The flange portion (7) is provided with a ring-shaped rib portion (20) that protrudes from the flange portion (7); the ring-shaped rib portion (20) is provided with a plurality of slits (22a and 22b) that extend from the flange portion (7) in the axial direction of the cylinder (6); and the flange portion (7) between the ring-shaped rib portion (20) and the cylinder (6) is inclined in a circumferential direction toward the slits (22a and 22b).