Method and apparatus for detecting deterioration in fuel injection amount of internal combustion engine

In a diesel engine structured to execute a low temperature combustion, such low temperature combustion and a fuel addition/after injection are inhibited (S 116, S118) upon detection of deterioration in fuel injection amount so as to execute a normal combustion. This makes it possible to eliminate the cause of combustibility fluctuation other than the deviation of the fuel injection amount. In spite of few chances of observing the state other than the low temperature combustion, accurate detection of deterioration in the fuel injection amount may be performed.