By setting an outer circumferential surface of a second hydraulic chamber of a cylinder device to be a thin portion, hydraulic pressure increases by receiving air pressure from the second pneumatic chamber, and the thin portion expands to clamp a cylinder. Then, by the increase of the second hydraulic chamber, expansion of the thin portion in an axial direction is suppressed and the thin portion expands in a radial direction, and thus, as means for suppressing expansion in the axial direction, the thin portion is fixed from both end sides thereof with an expansion stop bolt. The expansion stop bolt fixes a lid and a lid disposed so as to sandwich pressed portions on both end sides of the thin portion or both ends of the thin portion. Further, in order to prevent the thin portion from being shrunk by tightening with the expansion stop bolt, a spacer is disposed between the pressed portions on both end sides of the thin portion.