PURPOSE: To give a vibration to abrasive grains flowing down, disperse these abrasive grains in slurry uniformly, and shorten a span of lapping time as well as to improve the extent of accuracy of finishing, by lapping a workpiece as giving the ultrasonic vibration to the workpiece or the slurry.

CONSTITUTION: A single crystal 1 of LiTaO3 of about 3 inches in diameter and about 100mm in length is stuck plate 8, a Langevin vibrator 7 is set up in the lower part. And, the glass 8 is vibrated at about 50W/cm2, and a piano wire 5 is wound up on a roller 6 while adding slurry 3 on the single crystal 1, thus cutting takes place. If so, within about 8hr in machine time and about 5μm in straightness of a wafer, a surface becomes smoothed. On the other hand, if the same machining is performed without giving any vibration, the machining time is about 20hr and the wafer straightness comes to about 20W30μm. Thus, a workpiece is machined as giving an ultrasonic vibration whereby the machining time is reduced and, what is more, accuracy of finishing is improvable.