Provided is a unit which is capable of readily grinding and collecting a small amount of a sample which is difficult to be ground. The unit for grinding a sample comprises a club-shaped pestle and a mortar into which the pestle can be inserted, wherein the pestle comprises a roughened taper located at at least one end of its pestle body, wherein the mortar is a tube with an inverted frustoconical bottom, and the surface of the mortar where the taper of the pestle contacts the inverted frustoconical bottom when the pestle is inserted into the mortar is roughened, and wherein the angle of the taper with respect to the longitudinal centerline of the pestle is substantially equal to the angle of the inner wall of the inverted frustoconical bottom with respect to the longitudinal centerline of the mortar. The roughened taper and surface improve grinding efficiency.