A vibration electrode plate 112 is formed on the upper face of a silicon substrate 32 with an insulating coat film 35 interposed in between. An opposing electrode plate 113 is placed on the vibration electrode plate 112 with an insulating coat film interposed in between, and acoustic holes 40 are opened through the opposing electrode plate 113. Etching holes 36 and 104, each having a semi-elliptical shape, are opened through the vibration electrode plate 112 and the opposing electrode plate 113 so as to face each other longitudinally. A concave section 37 having a truncated pyramid shape is formed in the upper face of the silicon substrate 32, by carrying out an etching process through the etching holes 36 and 104. The vibration electrode plate 112 is held in the silicon substrate 32 by a holding portion 112 placed between the etching holes 36.