To prevent deterioration of combustion by determining deterioration of combustion due to a change of the fuel injection state even in the case wherein the fuel injection quantity is not changed by deposit.

Ignition time is advanced by a set quantity in an operating condition, in which burning condition is determined (S3), and a check whether a knocking margin is reduced or not is done (S4), and in the case wherein the knocking margin is reduced, the deposit eliminating treatment is carried out (S5), and ignition time is advanced again (S6). A check whether the knocking margin is improved or not is done (S7), and in the case wherein the knocking margin is improved, it is determined that the cause of deterioration of combustion is a change of the fuel injection state by deposit (S8), and in the case wherein the knocking margin is not improved, it is determined that the cause of deterioration of combustion is deterioration of particulate characteristics of the injected fuel outside an allowable range by abnormality of an injector (S9).