A plurality of strip-shaped base electrodes (27) are arranged in parallel to one another on the upper surface of a fixed substrate (22) and an electret (28) is formed on each of the base electrodes (27). The electret (28) has a width greater than that of the base electrode (27). The electret (28) covers the exposed surface of the base electrode (27). A movable substrate (23) is arranged to oppose to and in parallel to the surface of the fixed substrate (22) on which the electret (28) is arranged. The movable substrate (23) can be moved relative to the fixed substrate (22). The movable substrate (23) has an opposing surface on which strip-shaped opposing electrodes (32) are formed to oppose to the base electrodes (27), respectively.
