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We have applied a non–linear control system to a hydraulically–operated continuous valve timing control (C–VTC) now in the mainstream of variable valve actuation systems. The system applied this time is a sliding mode control (SMC), which is found of late in a number of applications. Hydraulic pressure serving as the driving source of the C–VTC and the mechanism of the C–VTC have non–linear characteristics. We have investigated certain problems which occur, influenced by this non–linearity, when using a PID controller for C–VTC control. Typical issues include a large program memory size because of the large number of control parameters, a resultant considerable number of man–hours required for adaptation, and the low compatibility of response performance both for large step operations and for small step operations. Furthermore, high machining accuracy is required for the mechanical components.