In an acoustic sensor, a diaphragm arranged on an upper side of a silicon substrate includes a back chamber, and an anchor supports the diaphragm. An insulating plate portion fixed to an upper surface of the silicon substrate covers the diaphragm with a gap. A conductive fixed electrode film arranged on a lower surface of the plate portion configures a back plate. The change in electrostatic capacitance between the fixed electrode film and the diaphragm outputs to the outside from a fixed side electrode pad and a movable side electrode pad as an electric signal. A protective film is arranged continuously with the plate portion at an outer periphery of the plate portion. The protective film covers the outer peripheral part of the upper surface of the silicon substrate, and the outer periphery of the protective film coincides with the outer periphery of the upper surface of the silicon substrate.