PURPOSE: To secure a largely stable closing force by attaching a pair of guiders to a body, and installing a pair of levers, coming into contact with the guiders for sliding motion, and whose one end is rotatably attached to a power transmission device and the other end is slidably engaged with a chuck jaw.

CONSTITUTION: When a chuck jaw 6 is closed from a state of being opened, a driving device is operated, a piston rod 1a is driven and each L-shaped lever 3 is rotated with a connector 7a as the center in using each guider 4 as a fulcrum. At this time, the lever 3 drives the chuck jaw 6 in the arrow direction via a connector 7b locked to a side end of the chuck jaw 6 of the lever 3. In succession, when the chuck 6 is opened from a state of being closed, the driving device is operated, the piston rod 1a is driven in the opposite direction, and then the lever 3 is rotated with the connector 7a as the center. At this time, the lever 3 drives the chuck jaw 6 in the opposite side and opens it via the connector 7b locked to the side end of the chuck jaw 6 of the lever 3.