A turbine, which is an essential portion of an air motor, is disposed midway along an air-intake passage communicating to combustion chambers of an engine. A generator is operably connected to an output shaft of the turbine. The generator has a function to control the rotational motion of the turbine in proportion to the magnitude of the generated electric energy, in addition to the original function of generating the electric energy. The generator is electrically connected to an air-intake control device. In other words, the air-intake control device includes a transistor, and potentiometer which includes a resistor and a sliding element. A positive electrode of generator is connected to an emitter of the transistor. Further, a base of the transistor is connected to an output terminal of the potentiometer, and a collector thereof to a battery. When the operation magnitude of an acceleration pedal is small, the current flowing through the base decreases. Therefore, the generator generates more electric energy. Along this condition, the generator regulates the rotation of the turbine in higher degrees. Consequently, the turbine becomes less rotatable. As a result, as it becomes difficult for the intake air to flow through the turbine, the flow volume of intake air to be introduced into the combustion chambers decreases.