A control apparatus is provided for ensuring a high-level control resolution and a high control accuracy even when a plant exhibits strong and varying non-linear characteristics with a varying sensitivity of a control amount to a control input. The control apparatus for controlling a cam phase through a cam phase varying mechanism comprises an ECU. The ECU calculates an SLD control input for controlling a cam phase to converge to a target cam phase in accordance with a predetermined control algorithm, and calculate a control input to the cam phase varying mechanism by modulating the SLD control input in accordance with a predetermined modulation algorithm. The ECU also sets an amplitude setting value in accordance with the cam phase, an engine rotational speed, an oil pressure, and an oil temperature.