PURPOSE: To prevent an error deciding of a fuel leakage resulting from an oil dilution generated following a deterioration of the starting property by converting a deciding value to compare with the difference of the learning correction coefficients in an idle operation area and in the other specific opera tion area, in a specific period decided depending on the starting property degree.

CONSTITUTION: The engine operation condition including at least the engine rotation frequency, the engine intake air flow amount, and the cooling water temperature is detected by a sensor A, and depending on the engine operation condition, a standard fuel injection amount is set by a means B. And learning correction coefficients to correct the standard fuel injection amount at each area of the engine operation condition are stored in a means C, and it is diagnosed whether the fuel is leaking or not from a fuel injection valve by a means D, depending on whether the difference of the learning correction coefficients in an idle operation area and the other specific operation area is higher than the deciding value or not. Furthermore, in a specific period decided depending on the starting property degree, the deciding value of the above means D is converted by a means E.