A pressing force sensor that includes a flat membrane piezoelectric element and a support. The flat membrane piezoelectric element includes a piezoelectric sheet having a piezoelectric constant. A first electrode is formed on a first main surface of the piezoelectric sheet and a second electrode is formed on a second main surface thereof. Long directions of the first electrode and the second electrode and a uniaxial stretching direction of the piezoelectric sheet form an angle of 45°. An opening portion having an elliptical section is formed on the support. The flat membrane piezoelectric element abuts the opening portion of the support. The support and the flat membrane piezoelectric element are disposed such that the opening portion is included within an area of the second electrode.