To provide a control device capable of controlling the position of a slider with high accuracy, for a gas pressure actuator causing the slider floated up-by a static pressure bearing to be driven by differential pressure by use of two double acting type servo valves.

The apparatus includes a computing device 20 for performing operations and outputting command values to the servo valves 22A and 22B by, sampling position detection signals transmitted from a position sensor 15 for detecting the position of the slider 13. The computing device performs the step of calculating the current estimated position, estimated speed, and estimated acceleration of the slider by use of both a command value which is outputted one, sampling period before and a slider position which is measured one sampling period before; the step of calculating command values for the two servo valves from a desired slider position and the calculated estimated position, estimated speed and estimated acceleration of the slider; the step of calculating a correction value for the neutral points of the servo valves as to differences among different actuators,,from the command value outputted one sampling period before, the estimated speed calculated one sampling period before, the estimated acceleration calculated one sampling period before, and the current estimated acceleration calculated; and the step of correcting the command value by use of the correction value.and imparting the corrected value to the two servo valves.