A rotary index device in which the rotating shaft is held surely and stably after indexing. In the rotary index device of a machine tool comprising a rotating shaft provided rotatably in the housing and having an end portion to which a circular table is secured, and a clamp device for holding the rotational angular position of a body of revolution after indexing by making a pressing force act in the radial direction of the rotating shaft to the body of revolution including the rotating shaft, the clamp device includes a piston member provided displaceably in the axial direction of the rotatingshaft within the housing and having a tapered face formed on the side face, a means interposed between the body of revolution and the piston member, receiving a pressing force in the axial directionof rotation of the rotating shaft from the piston member as it displaces and making the pressing force act on the body of revolution while converting into the radial direction of the rotating shaft, and a means for urging the piston member in the direction for generating the pressing force from the pressing force conversion means.