1. A control apparatus for an internal combustion engine configured to control the internal combustion engine which comprises: a three way catalyst disposed in an exhaust passage; an air-fuel ratio sensor disposed on an upstream side of the three way catalyst: and an oxygen concentration sensor disposed on a downstream side of the three way catalyst, said control apparatus comprising: a controller which is configured to perform first feedback (F/B) control in which an air-fuel ratio of a gas flowing into the three way catalyst converges to a target air-fuel ratio, based on an output value of the air-fuel ratio sensor, perform second F/B control in which the target air-fuel ratio or the output value of the air-fuel ratio sensor is corrected such that an output voltage value of the oxygen concentration sensor matches a target output value, learn an output characteristic corresponding to an individual variation of a detection limit of an oxygen concentration of the oxygen concentration sensor, correct a learning value of the output characteristic based on a terminal temperature of the oxygen concentration sensor when the output characteristic is learned, and set the target output value according to the learning value.