Abstract: An internal model control (IMC) automatically corrects the gain of the internal model control when the settings in the internal model are improper. The controller calculates and outputs a manipulated variable according to an IMC control algorithm, and an adaptive tuning section adaptively tunes the model gain of an internal model such that change rates of the controlled variable and the reference controlled variable used in said IMC section become the same value during a control response period in a transient state. When a transient state of control is detected, a transient state detection signal is output and a response start region is detected. The change rates of the controlled variable and the reference controlled variable in this response start region are determined. The corrected gain of the internal model is calculated from the change rates of the controlled variable and the reference controlled variable in the response start region.