Positioning control system and filter

A positioning control system has a disturbance observer for estimating a disturbance from a displaced distance of a cylinder of a controlled object, and feeding back an estimated disturbance. The positioning control system also includes a saturation element and a low-pass element disposed in a feedback loop including the disturbance observer, and a saturated value changer for changing a saturated value of the saturation element based on a deviation. The saturation element is disposed in a forward path of a positive-feedback minor loop, whereas the low-pass element is disposed in a feedback path of the positive-feedback minor loop.