PURPOSE: To make an automatic adjusting device applicable to an open loop system and a closed loop system, and further a cascade control system and to adjust an arithmetic control parameter in a short time during the automatic operation of a plant.

CONSTITUTION: This device is equipped with a plant identification part 16 which identifies a plant 14, inputting the output of a PID controller 12 inputting the deviation 11' between a target value 11 and the output 15 of the plant 14, with integral characteristics and an idle time model by using the input 13 and output 15 of the plant 14, and a PID parameter determination part 17 which calculates a proportional gain, an integral time, and a differential time as parameters of the PID controller 12 with the idle time model obtained by the plant identification part 16 and supplies them to the PID controller 12.