To attain a beautiful finish through dispensing with a skillful twisting action of arm, wrist, fingers by using one surface or double surfaces of a shell- like body of convex/concave curved surface such as a semi-circular, egg-shell type are dish-like surface, as a nail file surface.

A file action surface work is applied to a part of curved inner surface 1 of e.g. Semi-spherical shell 3 having a radius R1 and a part of curved outer surface 2 having a radius R2, otherwise abrasive material is fixed thereon. The curved lines Q0, Q1 of a curved file having a larger dia, than a curved surface P0, P1, P2, P3 are brought into contact with bent lines P0, L1, L2, L3, P3, and operation of file is carried out through left and right movement. Thereby a bent line including a curved line P0, N1, N2, N5, N6, N7, P3 is obtained. By repeating the work, an ideal curved line from P0, P1, P2 to P3 are obtained.