To provide a clamp device, having a small-sized and simple structure and accurately centering a work to quickly hold and release it.

Three radial grooves 4a are formed at equal spaces in an annular guide part 4 formed in the periphery of a work insert hole 1a in a casing 1. In each radial groove 4a, a clamp claw 7 is disposed slidably in each radial direction, and the outside of each clamp claw 7 is provided with a coiled spring 11 for energizing the claw outward. A cam wheel 6 is rotatably disposed in the casing outside the annular guide part 4. The inside of the cam wheel 6 is provided with a cam face 6a where the end of each clamp claw 7 comes into contact, which is formed so that each clamp claw 7 is equally slid in the radial direction according to the rotation of the cam wheel 6. A tooth part 6b is provided in the outer peripheral part of the cam wheel 6, and a rack 8 meshing with the tooth part 6b is disposed slidably in the casing 1.