PURPOSE: To uniformly grind the inner surface of a thin and long metal tube and dissolve the noise nuisance by using a mandrel which can be deformed or shifted, following the shape of the metal tube.

CONSTITUTION: The covering body 7 of the mandrel 1 of a burnishing tool which is inserted into a thin metal tube to be worked is formed from a member using a deformable elastic body such as synthetic resin and rubber. When burnishing work is applied on the inner surface of the thin metal tube by the burnishing tool, the covering body 7 of the mandrel 1 as elastic body is deformed, following the shape of the inner surface of the metal tube through a roller, and at the same time, a prescribed burnishing pressing pressure acts, and burnishing work can be applied. For example, the mandrel 1 is formed into a double structure, and the covering body 7 is compounded with the mandrel 1. The covering body 7 is made of nylon through molding or cutting and formed integrally with the mandrel 1 firmly by the means such as fusion adhesion, adhesion, and screw fixing, and fixed on a shaft 6.