Disclosed is a spindle device (5) for a machine tool such that draw bar (22) is inserted into spindle (20) which is rotatably supported in housing (12), and draw bar (22) is biased to a rear of spindle (20) by an elastic force of disc springs (23), resulting in tool holder (51) being clamped to spindle (20). Spindle device (5) is equipped with unclamping device (60) that comprises piston (63) which is provided in annular space (55) between spindle (20) and draw bar (22), and which, by means of fluid pressure, causes draw bar (22) to move toward the front of spindle (20) in resistance to the elastic force.