Intended is to unitize a clamp device for a rotation indexing apparatus for a machine tool, thereby facilitating the maintenance of the clamp device and the assembly of the indexing apparatus including the clamp device. The rotation indexing apparatus (1) for the machine tool comprises the clamp device (2) for clamping a clamp disc (11) by causing a clamp element (6) to exert a pressing force on the clamp disc (11), thereby to bring a rotary drive target member (4) such as a circular table into a clamped state. Of the individual members constituting the clamp device (2), the individual major members excepting the clamp device (11) to be mounted on a rotating shaft (5) are unitized.