An apparatus aims at learning a deviation from a reference injection characteristic of a fuel injector provided for a cylinder of an engine having an output shaft. In the apparatus, a detecting unit is configured to detect a minimum interval between temporally adjacent fuel injection periods in the plurality of fuel injection periods based on a behavior of the output shaft during execution of theplurality of fuel injections. The minimum interval maintains the temporally adjacent fuel injection periods to be non-overlapped with each other. A learning unit is configured to learn the deviation from the reference injection characteristic of the fuel injector based on the detected minimum interval.