Clamp devices configured such that dust does not enter insides of casings. The clamp devices each include the casing, and a clamp-side end section of a clamp arm is exposed from an opening of the casing. The clamp arm includes: a first circular arc section which is exposed from the casing when a workpiece is clamped by the clamp arm and is contained within the casing when the workpiece is not clamped by the clamp arm; and a second circular arc section which is exposed from the casing when the workpiece is not clamped by the clamp arm and is contained within the casing when the workpiece is clamped by the clamp arm. A first scraper and a second scraper are in sliding contact with the first circular arc section and the second circular arc section, respectively. The clamp device can also include a side surface cleaning scraper for making sliding contact with a side surface of the first circular section.