A resist mask 40, having penetrating holes 41, is formed on a rear surface of a silicon substrate 2. A planar shape of each penetrating hole 41 is formed to a shape with which its respective sides are curved to inwardly convex arcuate shapes with respect to a regular quadrilateral that is a target shape of a transverse section at a processing ending end side of a corresponding cavity 3. Next, dry etching is applied to the silicon substrate 2. The cavities 3 are thereby formed in the silicon substrate 2. As the etching progresses, a transverse sectional shape of each cavity 3 decreases in inward projection amounts of the respective arcuate shaped sides in the transverse sectional shape of the corresponding penetrating hole 41 of the resist mask 40. At a processing ending end side of the cavity 3, its planar shape is substantially the same shape as the regular quadrilateral that is the target shape.
