A control duty is obtained, which asymptotically makes the output of a control subject consistent with an output of a reference model. The output of the control subject simulates a dynamic characteristic of a variable valve timing controller, and the output of the reference model simulates an ideal input-output characteristic of the variable valve timing. The control duty makes a difference between the output of the reference model and an actual valve timing small enough. A parameter of a controller is consecutively corrected to make the difference small by means of a parameter correcting mechanism when the dynamic characteristic of the variable valve timing controller is varied due to a variation of an operating environment thereof so that the difference becomes large.