A failed manifold pressure sensor is identified in a control apparatus having two manifold pressure detecting systems as a failsafe. A first comparing unit outputs an abnormal signal when a difference ΔP between manifold pressures PMa and PMb detected respectively by two sensors is larger than a threshold value DP. A manifold pressure estimating unit estimates the manifold pressure from the engine revolution Ne, a throttle opening PLP, and an atmospheric pressure PAa. The difference calculating unit calculates a difference ΔPMb between the detected manifold pressure PMb and the estimated manifold pressure PMcal. The difference calculating unit calculates a difference ΔPMa between the detected manifold pressure PMa and the estimated manifold pressure PMcal. A second comparing unit compares the difference ΔPMa and the difference ΔPMb in response to the aforementioned abnormal signal. The failed manifold pressure sensor is specified according to which one of the difference ΔPMa and the difference ΔPMb is larger.