An injection quantity control apparatus for accurately performing an injection quantity learning process in a diesel engine creates an environment for obtaining a characteristic value in a one-to-one relationship with an actual injection quantity. A controller performs a one-shot injection operation for a cylinder of an engine while the environment is established. An injection command quantity is corrected based on an engine speed variation caused by the one-shot injection. After establishment of the condition and before the injection, the opening of a valve is controlled to be smaller than a reference and an opening of each of a diesel throttle and variable turbocharger is controlled to be larger than a reference. A composition of an air flowing into a combustion chamber is stabilized to ensure that the characteristic value detected after the one-shot injection is in a one-to-one relationship with the actual injection quantity.