To monitor the dead zone of a controlled system on-line and set a compensator which compensates the dead zone on-line by estimating an inverse function by using an estimated dead zone and fuzzy inference, and performing carry-up by using the inverse function.

A state monitor part 5 inputs the target value ym, the output (y) from the controlled system 3 and the output (u) from the compensator 2 to decide whether or not there is a limit cycle and also hold those data in time series. An inference part 6 extracts data needed to design the compensator 2 (e.g. a value compensating the dead zone of the controlled system 2 and value compensating hysteresis) by using fuzzy inference and supplies them to the compensator 2 while abrupt variation is suppressed by a linear filter 7. The compensator 2 inputs the necessary data and performs carry-up and automatically sets nonlinear input/output characteristics of the controlled system so that the input/output characteristics at a corresponding point of time are compensated.