PURPOSE: To perform secure selection in an extremely short time by chemically treating a crystal plate with fluoride regardless of its shape, and selecting a prescribed growth area on the basis of a crystal pattern appearing on the main surface of the crystal plate.

CONSTITUTION: A crystal plate by X-ray lamp photograpy is cut into small pieces along dotted lines 11 and 11', and 12, 12' and 12", and they are polished and then treated chemically with fluoride so that a crystal growth area appears on the main surface of the crystal plate. In figure A, a crystal piece having a +X area 2, an S area 5', and a Z area 4 is shown, and crystal boundary lines 6 are seen. In figure B, a crystal piece having only the Z area 4' is shown and no crystal boundary line 6 is seen. In figure C, a crystal piece having the Z area 4' and a -X area 3 is shown. For this purpose, the crystal piece is worked into small pieces until respective crystal areas can not be discriminated, and they are treated chemically with fluoride to select crystal pieces in each growth area according to said example.