PURPOSE: To prevent transmission of repulsion of a stop ring during work to a hand of an operator by arranging a worm part, which can be freely engaged with a worm gear part arranged in the base parts of a pair of arms, in an arm driving shaft and installing fingers which can be freely fitted in a hole of the stop ring.

CONSTITUTION: For expanding a diameter of a stop ring 3 fitted in a groove of a rod and the like, a ratchet claw 6 is released from a tooth of a ratchet gear 9 and a knob 10 is turned in the dotted line arrow direction, and then, arms 11, 12 are turned via a worm part 8 and a worm gear parts 21, 22. Subsequently, the ratchet gear 9 is advanced by means of the ratchet claw 6 with a movable handle 5 oscillated, the pair of arms 11, 12 are turned gradually so that a clearance between tips 1a, 2a of fingers 1, 2 is opened, and the stop ring 3 is fitted in the groove of the rod and the like after its diameter is expanded greatly. In this process, repulsion of the stop ring 3 is absorbed by means of the tooth of the worm part 8 so as not to be transmitted to the hand of an operator operating the knob 10 and the movable handle 5.