A rotary table device for a machine tool includes a main shaft that is rotatably supported in an insertion hole formed in a frame; a table that is secured to an end side of the main shaft, and on which a workpiece, serving as an object to be processed, is mounted; a clamp piston that is provided so as to be movable in an axial direction of the main shaft in the insertion hole; and a bearing that is provided between the table and the frame, and that directly supports the table with respect to the frame. The main shaft has a clamp surface facing the table. A pressure chamber is defined by the frame and the clamp piston. When an operating fluid is supplied to the pressure chamber, the clamp piston is caused to press-contact the clamp surface, so that the table is brought into a clamped state.