PURPOSE: To shorten the working time, the cleaning and drying time after work, namely to improve the working efficiency, with an easy arrangement, by using that which coats a lot of fine brasive grains on the surface of a free particle, as a free composite abrasive grain.

CONSTITUTION: The body which coats a lot of fine abrasive grains 1b on the surface of a free particle 1a is used as the free composite abrasive grain of the device made to perform the work with a free composite abrasive grain 1 and the solid gas 2 phase of a gaseous body being injected to the body 4 to be worked. Consequently, the works of the lap finishing that the surface roughness of the body 4 to be worked is improved, polishing, etc., can be facilitated.