Patent Document 1 and Patent Document 2 disclose a rotary table apparatus (rotation indexing apparatus) for a machine tool including a disc-type clamp device.  In the disc-type clamp device, a table (rotary drive target member) is fixed to a spindle (rotating shaft), and a clamp disc is attached to the rotating shaft such that the clamp disc cannot rotate with respect to the rotating shaft.  The clamp disc is pressed against a housing by a piston member so that the clamp disc is clamped between the housing and the piston member.  Thus, the clamp disc is clamped in a non-rotatable manner and rotation of the rotating shaft is stopped. 
According to the technique described in Patent Document 1, a ring-shaped (donut-shaped) space is formed between the housing (frame body) and a cylinder element (front-face plate) fixed to an end face of the housing, and a piston is guided by the space such that the piston is slidable in an axial direction of the rotating shaft.  In addition, according to the technique described in Patent Document 2, a ring-shaped space is formed in a cylinder element (cylinder), and a piston is guided by the space such that the piston is slidable in an axial direction of the rotating shaft. 
In the technique according to the related art disclosed in Patent Document 1, the space which is formed in the cylinder element or between the cylinder element and the housing and in which the piston member is fitted simply guides the piston element in the axial direction of the rotating shaft, and is open at one side thereof in the sliding direction of the piston member.  More specifically, when the cylinder element is detached from the housing, the piston member either remains in the housing or falls off from the cylinder element.  Therefore, it is not possible to detach the entire body of the clamp device as a unit.  In this structure, when it becomes necessary to perform maintenance for the clamp device, for example, when it becomes necessary to replace a worn piston member with another one and the clamp device is to be detached from the rotary table apparatus, each member of the clamp device must be detached individually.  Thus, cumbersome processes are necessary.  In addition, in the case where, for example, the clamp disc (brake plate), which is positioned deeper in the apparatus than the clamp device, is to be replaced with another one, the following processes are performed.  That is, first, each member of the clamp device is individually detached from the rotary table apparatus.  Then, the clamp disc is replaced with another one, and the clamp device is reassembled and attached to the rotary table apparatus again.  Thus, maintenance cannot be easily performed. 
In the structure according to the related art described in Patent Document 2, compression springs (springs) for returning the piston member from the clamped state to the original state is placed between the housing and the piston (in the main body of the rotary table apparatus).  Therefore, it is not possible to detach the entire body of the clamp device as a unit.  More specifically, when the clamp device is detached from the main body of the rotary table apparatus, the main component used in the clamping operation remains in the main body of the rotary table apparatus.  Therefore, the assembly state of the clamp device cannot be reestablished, and the clamp device cannot be used as an independent unit.  This will be a problem when, in particular, operation tests (performance tests) of the clamp device are performed before the clamp device is attached to the rotary table apparatus.     Patent Document 1: Japanese Unexamined Patent Application Publication No.  2006-95668    Patent Document 2: Japanese Unexamined Patent Application Publication No.  06-155231