A gas-pressure-driven apparatus includes a main body having a working chamber, a movable member moving relative to the main body with a pressure of the working chamber, a pressure sensor for detecting the pressure, a flow rate sensor for detecting the flow rate of the working gas. A method for controlling the apparatus includes calculating a pressure change amount from the detected pressure and an integrated flow rate from the detected flow rate when the pressure is changed in a state in which the volume of the working chamber cannot be changed, calculating an initial volume of the working chamber from the pressure change amount and the integrated flow rate, and calculating a post-change volume of the working chamber from the integrated flow rate and the initial volume after creation of a state in which the volume of the working chamber can be changed from the initial volume.