A clamp apparatus including first and second clamp arms, which are supported rotatably on a body, with a cover being installed to cover outer sides of the body. The cover includes openings through which portions of the first and second clamp arms are inserted and rotatably operated. An unloading tray is arranged in the body at a position below the openings in the direction of gravity. Spatter, which invades into the clamp apparatus from the openings when a welding operation is carried out on a workpiece that is clamped by the first and second clamp arms, is deposited on the unloading tray, and the spatter is eliminated by taking out the unloading tray to the exterior of the clamp apparatus.