Throttle opening control system for internal combustion engine

In a system for controlling opening of a throttle valve installed at an air intake system of an internal combustion engine mounted on a vehicle, a fully-closed value of the opening of a throttle valve is learning-controlled based on a detected opening of the throttle valve to update the learned fully-closed value, when operating state of the vehicle is under a prescribed operating state such as the operator rides the accelerator pedal; and the updating of the learned fully-closed value is inhibited in valve opening direction until the operating state of the vehicle moves outside the prescribed operating state and then again returns to the operating state. In addition, an amount for updating in the opening direction is set smaller than that in the closing direction. With this, it becomes possible to avoid engine speed destabilization and other various problems even if the learned value in effect during pedal riding is used for engine idling speed control.