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

CONSTITUTION: The PD controller 1 performs two proportional and differential operations for obtaining the manipulated variable (u)2 of servo system 4 based on the control deviation (e)8 of a controlled variable (y)3 as an observed value of the servo system 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 integral characteristics. Then an identification circuit 202 identifies the parameters of the model so that the differential value of the output of the model approximates the differential value of the controlled variable (y)3 as the observed value of the servo system 4. For the identification, three kind of dead times L are previously set and identification errors of the respective dead times L are found; and the dead time L corresponding to the minimum identification error is found by expressing the relation between the dead times L and identification errors as a quadratic expression and the final parameters of the model are determined.