1. A servo controller, comprising: a reference model unit which receives, as an input, a reference to generate a model output and a model input; a feedback control unit which receives, as inputs, a control output, which is output from a plant, and the model output to generate a feedback input so that the control output follows the model output; and an adder which adds the model input and the feedback input to generate a control input to be input to the plant, wherein the reference model unit comprises: a mathematical model which simulates characteristics of the plant, and generates the model output and a state variable from the model input; a model input memory which receives, as an input, the model input and which outputs, as a past model input, a stored signal of the model input of past; a model controller which receives, as inputs, the reference, the state variable, and the past model input to generate the model input; and a model controller determining unit which determines the model controller based on the reference, the state variable, and the past model input, and wherein the model controller determining unit determines a model gain which is a numerical vector based on the reference, the state variable, and the past model input, and wherein the model controller generates the model input based on a value that is obtained by multiplying the reference, the state variable, and the past model input by the model gain.