PURPOSE: To provide a chuck wherein a proper work alone is clamped by an operating member inserted into a work-transferring passage which judges whether the work is proper or not.

CONSTITUTION: A chuck body 4 connected to the head of a main shaft 3 in a body is provided with a turning arm 20 which may turn in a plane containing an axis of the main shaft. Moreover, an operating member 28 which is activated by a spring 22 in a direction engaging at an end with a work W carried into a supporting hole 5 in the chuck is linked to the other end of the turning arm 20. The operating member 28 is supported by the chuck body 4 so as to move toward the supporting hole 5. The operating member 28 moves into a work-transferring passage by turning the turning arm 20 with the advance of the work W, checking the advance of an improper work before reaching a clampling position by a clamp means 15.