With a precondition that a cooling water temperature sensor 16 and an intercooler exit gas temperature sensor 18 have been determined normal, whether an EGR cooler efficiency calculated is within a normal range is determined. When within the normal range, whether there is divergence between a calculation value of an intake temperature to be detected by an intake manifold gas temperature sensor 19 and an actual detection value of the sensor 19 is determined. When not in the normal range, whether the calculation value is excessively low is determined; and, just like the above, whether there is divergence between the calculation value and the actuation detection value of the intake manifold gas temperature sensor 19 is determined. Based on the determinations categorized, whether the EGR cooler 14, EGR gas temperature sensor 17 and intake manifold gas temperature sensor 19 are normal is determined.