1. An apparatus for controlling a modeled plant, the plant being represented by a model expression that is based on an input into the plant, an output from the plant, model parameters, and an estimated value for disturbance that is applied to the plant, the model expression including a term in which the estimated disturbance value is multiplied by at least one of the model parameters, comprising: an identifier for identifying the model parameters; a disturbance calculating unit for calculating the estimated disturbance value so that an error between a model output that is calculated based on the model expression and an actual output of the plant is minimized; and a control unit for calculating the input into the plant based on the model expression to control the plant, wherein the calculation of the input into the plant based on the model expression including the term in which the estimated disturbance value is multiplied by at least one of the model parameters enables the estimated disturbance value to be reflected in the input into the plant without being calculated by any of the model parameters.