PURPOSE: To adjust parameters of the PID controller in a state of a closed loop system and to find optimum parameters in a short time.

CONSTITUTION: The PID controller 1 performs three proportional, integral, and differential operations for obtaining the manipulated variable (u)2 of a process 4 based on the control deviation (e)8 of a controlled variable (y)4 as an observed value of the process 4 from a command (r)6; and the relation of an estimated controlled variable to the manipulated variable (u)2 is represented with a model consisting of dead time characteristics and primary delay characteristics and the relation of the estimated controlled variable to known disturbance (d)5 to the process 4 is represented with a model of primary leading/ delay characteristics. Then the outputs of both the models are added and an identifying circuit 202 identifies the parameters of the models so that the sum approximates a controlled variable (y)3 as an observed value of a process 1.