An SiO.sub.2 layer (3), a Ti layer (4), a Pt layer (5), a PLZT layer (6) and an IrO.sub.2 layer (7) are formed sequentially on an Si substrate (2). The IrO.sub.2 layer (7) functioning as a top electrode has a thickness of about 100 nm. Since the IrO.sub.2 layer (7) has conductivity lower than that of Pt or the like conventionally used as a top electrode and a skin depth deeper than that of Pt or the like, sufficient sensitivity can be attained by a thickness of about 100 nm.