To increase the number of blades of a multi-blade and prevent increase of flow-in loss due to a too narrow channel width between blades at blade leading edges for preventing circulating flow from deeply entering an impeller for forming suitable circulating flow at a blade end of a multi-blade centrifugal blower.

The multi-blade blower has multi-blade impellers alternately provided in a state of setting trailing side positions of large and small two kinds of impellers 3a, 3b having different code lengths at common position. The number of blades of the impeller can be sufficiently increased to prevent the circulating flow from deeply entering the blades to action effect of the good circulating flow of a former similar blade end side ring member by such a structure, a whole body of blade surfaces can be made effectively operated, and pressure boosting effect can be effectively increased by multi-blade. However, since the number of blades at an upstream side air suction part of an impeller in which main flow flows can be constructed in a few blade structure, channel width between each blade at that section gets wide and flow-in loss of the main flow can be reduced.