To obtain high precision without exerting adverse influence of a mechanical fault or disturbance noise by operating a mechanically movable part according to the deviation of the value of a level-superposition digital signal of a detection signal and a noise signal from a digital value indicating a command position.

When an opening extent detection signal indicating the opening extent of a throttle valve 10 is detected from a position detector 2, the opening extent detection signal is added by an adder 3 to, for example, a periodic noise signal outputted by a noise generator 4. The analog superposition signal outputted from the adder 3 is supplied to an A/D converter 5. This superposition signal is converted by the A/D converter 5 into a digital signal. A command value setter 7 which generates the command value indicating the target opening extent of the throttle valve 10 as a digital signal is connected to a subtracter 6, which subtracts the command value from the output digital value of the A/D converter 5. The accumulation of this deviation is obtained by a controller 8 as a controlled variable.