Abstract: A workpiece chucking device is strengthened in its workpiece chucking force. The workpiece chucking device comprises a diaphragm 14 having chucking claws 35 in the front of a base 12 which will rotate along with a spindle 11 of a machine tool, a piston which will be moved back and forth by switching fluid fed into a hollow chamber 15 of the base, a cylindrical member 21 inserted in a center of the piston and held to the base, first balls 24 fitted into even-numbered ones of a plurality of transmitting holes formed in the cylindrical member, as well as second balls fitted into odd-numbered ones of the transmitting holes, and a recessed fitting portion 29 provided in an outer circumferential surface of a slider 26 which is inserted into the cylindrical member and connected to the diaphragm so that parts of circumferential surfaces of the first and second balls will be fitted into the recessed fitting portion 29.