To provide a gas pressure actuator capable of compensating dynamic characteristics change according to a slider position and stably controlling a slider in a stroke.

A control computing device 20 executes a step that the position of the slider 13 indicated by a position detecting signal from a position sensor 15 is differentiated to calculate the speed of the slider and acceleration is calculated through differentiation of the calculated speed, acceleration is calculated; a step that by using a slider target position, a slider position, a slider speed, and slider acceleration, position command values to be outputted to servo amplifiers 21A and 21B are calculated; and a step that based on calculated respective position command values, computation to compensate for the pressure change contents of pressure chambers 16A and 16B due to the position change contents of pressure chambers 16A and 16B due to the position change of a pressure received plate 17' in the slider is effected, and respective compensated position command values are outputted to two servo amplifiers.