1. An internal combustion engine output torque control system comprising: first means for determining a basic spark advance angle based upon predetermined control parameters including an intake air flow rate, the first means being operable to determine an ignition timing in accordance with the relationship between said spark advance angle and the reference position of the engine crankshaft; second means for calculating a difference between the torque demanded for the engine and the actual torque actually produced by the engine based on engine operating parameters including engine revolution speed, the second means being operable to determine said engine demand torque based upon said engine revolution speed and said intake air flow rate and said actual torque based upon the demand torque and the actual torque measured during the last cycle of the engine, and to determine a correction value used in correcting the ignition timing in a manner to reduce the difference between the actual torque and the demand torque; and third means for correcting the ignition timing determined by the first means for the correction value determined by the second means to determine a corrected ignition timing, the third means being operable to generate an ignition signal at a predetermined timing corresponding to the corrected ignition timing.