1. A stop control system for an internal combustion engine, which controls a stop position of a piston of the engine to a predetermined position during stoppage of the engine by controlling an intake air amount, comprising: an intake air amount-adjusting valve for adjusting the intake air amount; rotational speed-detecting means for detecting a rotational speed of the engine; intake air amount control means for controlling said intake air amount-adjusting valve toward a closed side when a command for stopping the engine is issued, and thereafter when the detected rotational speed of the engine becomes lower than a stop control start rotational speed, controlling said intake air amount-adjusting valve toward an open side; final compression stroke rotational speed-obtaining means for obtaining the rotational speed of the engine in a final compression stroke immediately before the engine is stopped, as a final compression stroke rotational speed; correlation determining means for determining a correlation between the stop control start rotational speed and the final compression stroke rotational speed, based on the stop control start rotational speed and the final compression stroke rotational speed obtained when said intake air amount-adjusting valve has been controlled toward the open side based on the stop control start rotational speed; and target stop control start rotational speed-setting means for setting a target stop control start rotational speed that serves as a target of the stop control start rotational speed, based on the determined correlation and a predetermined final compression stroke rotational speed for stopping the piston at the predetermined position.