PURPOSE: To efficiently perform a recessed groove forming work by supporting freely rotatably in the state of pressing a pair of burnishing rollers to the external side by press-fitting the tapered face of an expanded roller to the tapered face of a pair of burnishing rollers.

CONSTITUTION: An expanded roller shaft 12 is fixed to the tip of the 1st holder 11 whose rear end is fixed to the head of a lathe, etc., and an expanded roller 13 is supported freely rotatably by this shaft 12. Moreover, a burnishing roller shaft 15 is fixed to the front part of the 2nd holder 14 which is freely slidably fitted into the 1st holder 11 and a pair of burnishing rollers 16 are arranged freely rotatably on this shaft 15. The burnishing roller 16 is supported freely rotatably in the state of pressing it to the external side by press-fitting the tapered faces 13a, 13b of the expanded roller 13 to the tapered faces 16b', 16b' of these burnishing rollers 16, and burnishing is executed on the side face of the external side of a pair of recessed grooves.