A clamping device in a main shaft driving device for a machine tool, which rotationally drives a main shaft, wherein the main shaft is rotatably supported by a frame. The clamping device includes a clamp piston, a pressure chamber and an operating fluid supplying mechanism, wherein the clamp piston has a first pressure-receiving surface and a second pressure-receiving surface, the first pressure-receiving surface receiving the spring force of a spring member, the second pressure-receiving surface receiving pressure of an operating fluid, wherein the operating fluid supplying mechanism includes a switching portion that selectively supplies the corresponding operating fluid from a common operating fluid supply source to at least one of a first pressure chamber and a second pressure chamber.