To increase a frequency of the execution of the abnormality diagnosis of a variable air intake system, and to improve the accuracy of the abnormality diagnosis while satisfying a requirement for cost reduction.

The abnormality diagnosis that determines the occurrence of the breaking of an electric circuit based on a voltage level of the electric circuit of the variable air intake system 19 is executed in such an operating state that the degree of the opening of an air intake passage switching valve 23 is not changed (for example, in such an operating state that the air intake passage switching valve 23 is maintained in a valve-opening state). It is thus possible to increase the frequency of the execution of the abnormality diagnosis by substantially surely executing the abnormality diagnosis of the variable air intake system 19 every time an engine 11 is operated. Further, adjustment work for the abnormality diagnosis becomes unnecessary so that the cost reduction can be achieved. In the operating state that the degree of the opening of the air intake passage switching valve 23 is not changed, it is not necessary to change a driving voltage or a driving current of a motor. Therefore, the abnormality diagnosis based on the voltage level of the electric circuit can be executed accurately.

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