A clamping device in a main shaft driving device for a machine tool, in which the main shaft driving device rotationally driving a main shaft by a driving device, the clamping device includes: a braking member provided at a side of the frame and including a pressure-receiving surface and a braking portion; a pressure chamber formed by the pressure-receiving surface and the frame; and an operating fluid supplying mechanism provided for supplying the operating fluid from an operating fluid supply source to the pressure chamber. The press-contact force is caused to act upon the main shaft. The operating fluid supplying mechanism includes a switching portion that selectively supplies the operating fluid. The braking member is formed so that the braking portion causes the press-contact forces that differ from each other.