PURPOSE: To reduce the machining cost without requiring skill and make the mirror finishing of undercuts and details possible by a method wherein binder added with abrasive grains under molten state is filled in the cavity of a mold and, after the solidification of the binder, supersonic vibration is applied to the mold.

CONSTITUTION: Binder A added with abrasive grains under molten state is filled through a sprue 9, a runner 9 and a gate 7 in a cavity 1 so as to solidify it under the state being filled in the cavity. Next, by bringing a supersonic vibration generating device 14 into contact with a mold, the mold, the binder A in the cavity 1 and the abrasive grains are vibrated. Thus, the difference amount the vibration propagations of the mold, the binder A and the abrasive grains develops due to the difference among their material properties, resulting in allowing to develop relative and vertical micro-vibrations at the boundary surface between either two of them. As a result, the inner surface of the cavity 1 or mold surface is polished with the abrasive grains so as to be mirror- finished. This method necessitates no skillfullness and realizes mirror finishing in a short time and further can easily mirror-finishes under cuts and details.