PURPOSE: To enable two slave controllers to switch the cascade close state with no bump against a master controller while keeping a ratio between both slave controllers by placing two multipliers between the master and slave controllers respectively to set the ratios among them.

CONSTITUTION: The signal with a slave controller 6 of the side A switched from a cascade open state to a cascade close state is converted into a one-shot signal by a one-shot signal generator 10. Then '1' is written into the ratio constant of the side A by a switch 8 of the side A. Then the output of a master controller 2 is set at the present set value of the controller 6 by a switch 11. Furthermore the value obtained by dividing the hitherto set value of a slave controller 15 of the side B by the present set value of the controller 6 is defined as a comparison constant 13 of the controller 15. Thus it is possible to continue the control based on the hitherto value while keeping a ratio between both controllers 6 and 15.