PURPOSE: To improve the acceleration performance and the fuel economy, in an internal-combustion engine where a mechanical supercharger to be driven mechanically through a clutch is provided to an intake tube, by controlling such that the clutch is engaged upon elapse of predetermined delay time when transiting to accelerating condition.

CONSTITUTION: A mechanical supercharger (Root's pump) 3 is provided to the intake tube 2a of internal-combustion engine and coupled through clutch means (electromagnetic clutch) 4 to the rotary shaft 2b of internal-combustion engine to be driven mechanically while the clutch means 4 is controlled through clutch control means 5 with correspondence to the engine load. Here, means 6 for detecting transition to the accelerating operational region requiring no supercharge is provided. If such speed change characteristic as considering the economy important is selected for the automatic transmission 1 through manual selecting means 1a, it is controlled to engage the clutch means 4 upon elapse of delay time set through delay means 7 when transition to said accelerating operational region is detected.