To make a piezoelectric element thinner than the thickness of manufacturing limit which was considered to be difficult in the past by using two, upper and lower lapping plates, than is, a lower lapping plate which is located on the lower surface of a piezoelectric element polished material and an upper lapping plate located on a first machining assisting tool.

A first machining assisting tool 11 uses two, upper and lower lapping plates, the upper lapping plate 18 and the lower lapping plate 17. The lower lapping plate 17 machines a cylindrical shape or the other shape with quartz, and the upper lapping plate 18 polishes the back face where a groove or a stepped part of a ring shape, a quadrangle or the other shapes is formed. By a both-sided polishing machine, a very thin work piece is polished. A second machining assisting tool 19 and the quartz plate 13 may be disposed in the reverse state to perform polishing.