A motorcycle supercharger (42) pressurizes intake air (I) for a combustion engine (E). The supercharger (42) includes a centrifugal impeller (50) and an impeller casing (52) which covers the impeller (50). The impeller casing (52) includes an outer peripheral wall (84) located radially outward of the impeller (50) and a side wall (86) located axially outward of the impeller (50). Reinforcing first and second side wall ribs (94, 96) are provided at the side wall (86) of the impeller casing (52).