To miniaturize an entire opening/closing chuck by suppressing enlargement of a chuck body for space securement for providing a through hole for mounting a fixing screw, in the opening/closing chuck in which a pair of gripping members are opened or closed in parallel with the driving direction of each of a pair of cylinder mechanisms by the cylinder mechanisms arranged in a chuck body.

A guide member 4 and the chuck body 1 are provided with the through hole 28 for passing the fixing screw that penetrates the guide member 4 and the chuck body 1 and extends to a surface facing to the arranging surface side of the pair of gripping members 3 and 3. Each gripping member 3 has an insert hole 29 for inserting the fixing screw from the arranging surface side into the through hole 28.