A mechanical supercharger includes a housing, first, second and third shafts rotatably supported on the housing, respectively, a first timing gear disposed on the first shaft, a second timing gear disposed on the second shaft and engaged with the first shaft timing gear, a first rotor disposed on the first shaft, a second rotor disposed on the second shaft and engaged with the first rotor, a first pulley disposed on one end of the first shaft, an electromagnetic clutch interposed between the first shaft and the first pulley and being selectively intermittent a transmission of driving force between the first shaft and the first pulley, a reduction gear disposed on one end of the third shaft through an one-way clutch and engaged with the second timing gear, a second pulley disposed on the other end of the third shaft and a driving force transmission means for transmitting driving force of engine to the first and second pulleies so that the first and second pulleies are simultaneously rotated in the same direction. Thereby, it is possible to reduce the number of the additional parts for changing the supercharging quantity in response to the condition of the engine and it is possible to prevent the unnecessary increasing of the rotating mass of the rotation groups which are rotated for supercharging.