To clamp a clamped member having dispersion in a width shape in a clamping direction while automatically centering it without changing clamping force, with a simple constitution.

At least a pair of cylinders 12, 13 are arranged to be opposed by interposing a clamped member 1, an unclamp side port of one cylinder 12 and a clamp side port of the other cylinder are connected to communicate by a pipe 42 connected to a tank 18, and a two-directional pump 45 is provided between the pipe 42 and the tank 18. In this way, the two-directional pump 45 is controlled so that a difference between rod displacement amounts of each cylinder 12, 13 is a preset value.