PURPOSE: To clamp a forging die firmly and quickly, by constituting a clamping device of the forging die, of a clamper having a tapered surface, which is capable of moving up and down, and back and forth, and a pair of cotters provided on the side of the clamper.

CONSTITUTION: When clamping a forging die B, a truncated pyramid-like wedge clamper 1 having a tapered surface is made to press-contact with the forging die B by its tapered surface, by operating a clamp shunt cylinder 41. Subsequently, by operating a cotter shunt cylinder 21, a cotter 2 is moved to the back from both sides of a holding plate 4. In this state, a clamp cylinder 3 is operated, the wedge clamper 1 is made to ascend by drawing upward a cylinder rod 31 and a clamp rod, and the forging die B is firmly clamped so as to push-separate the forging die B and the cotter 2.