1. A control apparatus for an internal combustion engine, comprising: a throttle valve that is arranged in an intake passage of the internal combustion engine; throttle opening control means for controlling a throttle opening of the throttle valve to change an effective opening area of the intake passage to variably control an amount of intake air to the internal combustion engine; means for detecting an actual throttle opening of the throttle valve; operating state detection means for detecting an operating state of the internal combustion engine, the operating state detection means including intake air amount detection means for detecting the amount of intake air to the internal combustion engine, atmospheric pressure detection means for detecting a pressure at an atmospheric side of the throttle valve as an atmospheric pressure, intake pipe internal pressure detection means for detecting a pressure at an internal combustion engine side of the throttle valve as an intake pipe internal pressure, and intake air temperature detection means for detecting an intake air temperature at the atmospheric side of the throttle valve; target intake air amount calculation means for calculating a target amount of intake air based on the operating state of the internal combustion engine; target effective opening area calculation means for applying the target amount of intake air, the atmospheric pressure, the intake pipe internal pressure, and the intake air temperature to a flow rate formula for a throttle type flow meter to calculate a target effective opening area of the throttle opening control means; target throttle opening calculation means for using a correlation map between the effective opening area of the throttle opening control means and the throttle opening of the throttle opening control means, which are suited to each other in advance, to calculate a target throttle opening from the target effective opening area; actual effective opening area calculation means for applying the amount of intake air, the atmospheric pressure, the intake pipe internal pressure, and the intake air temperature to the flow rate formula for the throttle type flow meter to calculate an actual effective opening area of the throttle opening control means; and learning throttle opening calculation means for using the correlation map to calculate a learning throttle opening from the actual effective opening area, wherein the throttle opening control means includes throttle opening learning value calculation means for calculating a throttle opening learning value based on a deviation between one of the actual throttle opening and the target throttle opening, and the learning throttle opening, and controls the throttle opening by a learning corrected target throttle opening obtained by correcting the target throttle opening with the throttle opening learning value, and wherein the throttle opening learning value calculation means calculates the throttle opening learning value as a value composed of a real-time learning value updated in real time and a long time learning value corresponding to each learning region according to an effective opening area axis point of the correlation map, and updates and stores the real-time learning value and the long time learning value based on a magnitude relation between values respectively obtained by adding the long time learning value to throttle openings respectively indicated by two effective opening area axis points of the correlation map, between which lies the actual effective opening area, and the actual throttle opening when the throttle opening learning value is to be calculated.