To provide a clamper adapted to clamp a work with an arm by converting linear motion of a cylinder to a rotary motion by a toggle mechanism.

Projecting parts 67a, 67b for release are wrapped by a metal-made top cover 200, preferably with a top cover coated with a material having excellent sliding performance. Accordingly, at the time of arc welding, a spatter is inhibited from adhering, and even supposing that it adheres, it can be easily removed. When the arm 20 is to be unlocked by the projecting parts 67a, 57b for release, a tongue piece 210 of the top cover 200 is lifted up to be opened, and the projecting parts 67a, 67b for release are pushed down.