The chuck has a piston (25) which is moved in one direction and the fingers (18) are opened if the fluid under pressure is introduced into the second pressure chamber (28). The piston is moved in the opposite direction if fluid under pressure is fed into the first pressure chamber (27). A leading end surface of the rod (26) is used as the pressure take-up surface to create a force acting in a direction to close the fingers by lengthening the first pressure chamber to a point which encompasses the end of the rod.
