To shorten a frequency-adjusting working hour in a crystal-wafer level.

The frequency of a crystal piece is adjusted on the thickness of a crystal plate constituting the crystal piece at two stages by means of a partial wet-etching method. In the first stage, the thickness in a plurality of crystal pieces is roughly adjusted in a mass through the partial wet etching. Subsequently, the thickness is subjected to the partial wet etching for fine adjustment at each of small amount of crystal pieces, or variations are roughly adjusted to get an even constitution in the thickness of the crystal piece which is put to fine adjustment simultaneously in the first stage of the partial wet etching. Thereafter, the thickness of the crystal piece is put to the fine adjustment through the partial wet etching in the mass belonging to the group of equal thicknesses.