This invention comprises two electrode plates 1a and 1c which are arranged in parallel to each other, and a middle electrode plate 1b. One end of an electrostatic capacity member 2 serves as a fixed portion 3 while the other end thereof serves as a movable portion 4, and both end portions of each of these electrode plates 1,1 are provided with hinges so that the movable portion 4 is movable in parallel relative to the fixed portion 3. Additive capacity is further provided by the insulating plate 6 to improve the linearity of the detection output.