To provide a structurally simple clamping device for quickly changing a clamping width, and reducing the number of part items.

A guide hole 24 of a lower part 21a of a body 21 does not form a female screw part, and a screw bar 28 is movably inserted into this guide hole 24. A hexagonal nut 26 having a female screw part 26a for threadedly engaging with a male screw part 28a is rotatably arranged in an outer peripheral part of the screw bar 28. When adjusting the clamping width B, the hexagonal nut 26 is put in a free state by separating from a hole part 25 being a rotation regulating part, and the hexagonal nut 26 is moved in the shaft direction to the screw bar 28 by rotating the nut. When the hexagonal nut 26 moves up to a proper position, the hexagonal nut 26 is housed in the hole part 25 by pulling the screw bar 28, and a position of a second butting part 29 is finely adjusted by rotating the screw bar 28 without rotating the hexagonal nut 26.