To provide an effect substantially equivalent to the case with a retention duty learned in the whole temperature range without learning a retention duty in the other temperature range by only learning a retention duty required for retaining actual valve timing in a constant state within a predetermined temperature range in a variable valve timing control system.

At the design-production stage of a product, retention duty reference characteristic data defining a relation between oil temperature or temperature correlated therewith (such as cooling water temperature) and the retention duty are previously stored in a nonvolatile storage means such as an ROM 41 in an ECU 24. The retention duty is learned within the predetermined temperature range (for example, a temperature range equivalent to oil temperature or cooling water temperature after warming-up). The retention duty in the other temperature range is set based on the learned value of the learned retention duty within the predetermined temperature range and the retention duty reference characteristic data read out from the ROM 41 or the like.