open position that is while the three way solenoid valve 58 is held in the second position the area of opening of the throat 52 is inversed to permit increased air flow through the return passage 46 whereby the degree of supercharge applied to the engine is reduced to the contrary while the three way solenoid valve 58 is held in the first position the amount of air flowing through the return passage 46 is reduced with an increased degree of supercharge the position of the three way solenoid valve 58 is changed by the function of a valve drive circuit 60 a vibration sensor 62 is provided which is adapted to detect vibrations and provide a signal representative of detected vibrations the vibration sensor 62 may be in the form of a resonance vibration sensor having a resonance frequency equal to that of knock occurring in the engine the output of the vibration sensor 62 is coupled to a knock intensity detecting circuit 70 which controls the operation of the valve drive circuit 60 in accordance with detected knock intensity in view of the known fact that the tendency of occurrence of knock becomes lower with a smaller degree of supercharge applied to the engine the knock intensity detecting circuit 70 controls through the valve drive circuit 60 the three way solenoid valve 58 such that the degree of supercharge can be reduced to a suitable level allowing light knock leading to high fuel economy and high engine output characteristics when the.