1. A control apparatus for controlling a controlled variable of a controlled object by a control input, comprising: controlled variable-detecting means for detecting the controlled variable; reference parameter-detecting means for detecting a reference parameter of the controlled object other than the controlled variable of the controlled object; target controlled variable-setting means for setting a target controlled variable serving as a target to which the controlled variable is controlled; and control input-calculating means for calculating a first input value for feedforward control of the controlled variable, according to the reference parameter, using a correlation model representative of a correlation between the reference parameter and the first input value, calculating a second input value for performing feedback control of the controlled variable such that the controlled variable is caused to converge to the target controlled variable, with a predetermined feedback control algorithm, and calculating the control input based on the first input value and the second input value, wherein said control input-calculating means comprises: error parameter-calculating means for calculating an error parameter indicative of a control error to be compensated for by the first input value, based on the controlled variable and the target controlled variable; model-modifying means for calculating a plurality of modification values respectively associated with a plurality of regions formed by dividing a region within which the reference parameter is variable, with a predetermined control algorithm, such that the error parameter becomes equal to a predetermined target value, and modifying the correlation model using the plurality of modification values; and first input value-calculating means for calculating the first input value using the modified correlation model.