To provide a quartz oscillator having a higher natural frequency by performing work more thinly by preforming a thin recessed part into a recessed shape in a central part of a quartz plate to be worked.

A thin recessed part 3 is preformed into a recessed shape in a central part of a quartz plate 2 to be worked. This is ground as a work raw material 1. Grinding pressure acts on a part to be ground. A part having no recessed surface on the opposite side is normally ground by grinding pressure acting on the plane side. On the contrary, a part having a recessed surface on the opposite side deflects on the recessed surface side by the grinding pressure. Grinding is not normally performed by this deflection, and a largely deflected part is ground in response to a deflection quantity so as to reduce a stock to be removed. Thus, the stock to be removed is continuously reduced toward the center of a recessed surface, so that when the grinding pressure is released after finishing the grinding, a convex lens shape is formed in a part being originally a plane on the opposite side of the recessed surface.