PURPOSE:To increase the effective torque of an engine in the high engine revolution range by setting the revolution ratio between an engine output shaft and a complex supercharger so that the revolution speed in the compulsory revolution by the engine output shaft becomes larger than the self-traveling revolution speed in the case when a clutch is cut off. CONSTITUTION:When a clutch 26 is connected, a cell 23 is compulsorily revolution-driven with the revolution ratio corresponding to the diameter ratio between a pulley installed onto an engine output shaft and a pulley 27 on the cell 23 side. The number of revolution of the cell 23 in the case when the clutch 26 is connected is set so as to be larger than the self-traveling revolution speed of the cell 23 in the case when the clutch 26 is cut off. Therefore, a complex supercharger 21 can be operated efficiently over a wide revolution region of an engine with an exceedingly simple constitution, and the driving resistance of the complex supercharger 21 is reduced markedly in particular in the high engine revolution region, and the effective torque of the engine can be increased.
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