a supercharged engine knock control system of the class in which spark advance is retarded by a retard correction value varying in response to an occurrence of knocking is improved in such a manner that the retard correction value computed under engine conditions which do not accurately reflect the octane rating of the fuel is not taken into account in determining the boost pressure control . to this end , the knock control system comprises a memory for storing predetermined engine load threshold values that vary depending on engine speed . boost pressure is lowered when the actual engine load is greater than the engine load threshold value and when the retard correction value is greater than a reference value .