PURPOSE: To improve the lapping quality and improve the work efficiency by concentrically fixing two pull cones to one pull rod, and concurrently moving both support legs of a collet in the radial direction by the same quantity via the shift of the pull rod in the axial direction.

CONSTITUTION: When a pull rod 9 is pulled upward in the axial direction by an action from the outside, two pull cones 10, 11 are concurrently moved by the same quantity, both support legs 13a, 13b of a collet segment 13 are moved in the radial direction by the same quantity, a hollow rubber tube 3 is expanded, and piston rings 4 coupled with the outer periphery of the hollow rubber tube 3 are expanded. The size of the interval between both support legs 13a, 13b does not affect the action to uniformly expand the piston rings 4, many piston rings 4 can be coupled when the interval is increased, and many piston rings 4 can be concurrently expanded to uniformly press a lapping sleeve 12.