PURPOSE: To detect that a workpiece is chucked precisely by means of chuck jaws so as to precisely and smoothly transfer the workpiece to the next process.

CONSTITUTION: When a piston member 44 is slid forward to a cylinder member 43, respective chuck jaws 55 connected to the piston member 44 are moved to the limit position on the outside in the radial direction by means of a connecting body 48 and an operation lever 56. Subsequently, the piston member 44 is slid backward when a fact that a workpiece is inserted into the respective chuck jaws 55 is detected by means of proximity switches 61a, 61b in a workpiece position sensor means, and the workpiece is chucked by means of the respective chuck jaws 55. In this way, one proximity switch 62b in a chuck sensor means is operated, while the other proximity switch 62a is not operated. If the workpiece is not precisely chucked by means of the respective chuck jaws 55, both of the proximity switches 62a, 62b are operated or not operated.