To diagnose abnormality of an intake system in a specific cylinder of a variable cylinder engine.

An intake air amount sensor capable of changing an output value by responding to intake air in each cylinder and having high response property is provided in an intake passage being common among cylinders and amount of intake air in each cylinder is calculated based on an output value Vap of the intake air amount sensor to determine abnormality of an intake system in a predetermined cylinder based on the calculated amount of intake air in the predetermined cylinder. For example, if the predetermined cylinder is a halt cylinder #4, amount of intake air in the halt cylinder is compared with a predetermined value (a Vth integrated value) to determine abnormality of an intake system in the halt cylinder. When it is determined that an intake system in a certain halt cylinder is abnormal, the halt cylinder is switched to an operation condition.