An object of this invention is to accurately detect a sensitivity abnormality of an in-cylinder pressure sensor over a wide operating range without using other sensor outputs or the like. Based on only the output of an in-cylinder pressure sensor 44, an ECU 50 acquires a first parameter that is affected by an output sensitivity of the sensor, and a second parameter that is not affected by the output sensitivity. Specifically, the ECU 50 acquires a heat release quantity PVκ as the first parameter, and acquires an indicated torque ratio (A2/A1) as a second parameter. The ECU 50 also calculates a determination coefficient α that is a ratio between the first and second parameters, and determines that the output sensitivity of the in-cylinder pressure sensor 44 is abnormal when the determination coefficient α deviates from an allowable range. Thus, even without utilizing another sensor output or the like in an auxiliary manner or previously preparing a large amount of data or the like, a sensitivity abnormality of the in-cylinder pressure sensor 44 can be easily detected over a wide operating range.