PURPOSE: To increase a clamping force and to prevent the damage to a clamping device owing to deviation in an axis.

CONSTITUTION: A vertically extending through-hole 2 is formed in a base 1 and a female screw 7 for clamping of a mold 6 is positioned facing the through- hole 2 from below. A male screw 10 engaged with the female screw 7, a press part 11, and a part 12 to be rotated are arranged, in the order, from below to a clamp rod 9 inserted in the through-hole 2. A piston 21 is inserted in a housing 3 fixed to a base 1 and a pneumatic type impact trench 14 is supported to a piston rod 24 protruded up from the piston 21. During clamping, compressed air is fed to a lowering drive chamber 22 to lower the piston 21 and the part 12 to be rotated is rotated by the impact trench 14. In which case, The clamp rod 9 is engaged with the female screw 7 and lowered in a screwing-around manner and the base 1 is pressed from above by a press part 11.