PURPOSE: To provide a efficient grinder in which a flexible grinding or polishing material is removably planted in a holder removably inserted in a rotary disc and the curved surface of the grinding or polishing material is caused to generate a machining surface by the rotation of the disc.

CONSTITUTION: A rotary disc 2 is mounted on a rotary shaft by a fitting part 1. A holder 5 is removably inserted in a fitting part 3. A grinding or polishing material 6 is removably planted in the holder 5. The material 6 is placed at intervals on a curved surface around the rotary disc 2 so that the curved surface of the material 6 is caused to generate a macining surface by the rotation of the disc 2. An elastic member 12 is provided on the peripheral surface of the disc 2 to control the curvature of the material 6.