1. A control apparatus for controlling a controlled variable of a controlled object using a control target model of a discrete-time system which represents a dynamic characteristic of the controlled object, wherein the control target model includes, as model parameters, at least one set of model parameters, each set being formed by a plurality of model parameters defined to be required to satisfy a predetermined restraint condition, and the predetermined restraint condition is that a predetermined one model parameter of the plurality of model parameters defines the other model parameters, the control apparatus comprising: identification means for arranging the control target model such that a group of terms which are not multiplied by at least one model parameter including the predetermined model parameter and a group of terms which are multiplied by the same are on respective different sides of the control target model, and, assuming that one of the different sides of the control target model represents a combined signal value and the other of the different sides of the control target model represents an estimated combined signal value, identifying onboard the at least one model parameter such that an error between the combined signal value and the estimated combined signal value is minimized; and control input-calculating means for calculating a control input for controlling the controlled variable, using the identified at least one model parameter and a control algorithm derived from the control target model.