To surely prevent hard particles for modifying a surface of a workpiece from scattering to the circumference while smoothly and surely supplying the particles to an area in which the processing of the workpiece is performed.

A rotary tool 2 is disposed oppositely to the surface of the round bar-like workpiece 1 through a minute gap, the workpiece 1 is longitudinally moved while being rotated around an axis, and during that time, dispersion liquid 6 with hard particles 6a dispersed therein is supplied from a dispersion liquid supply pipe 12 through which the rotary tool 2 is inserted through a circulation pipe 19 while rotating the rotary tool 2, whereby the dispersion liquid 6 is supplied to between the outer circumferential surface of the workpiece 1 and a working surface 2a of the rotary tool 2 to modify the surface of the workpiece 1 by the pressing force of the hard particles 6a.