Provided is an acoustic sensor capable of improving an S/N ratio of a sensor without preventing reduction in size of the sensor. A diaphragm 43 as a movable electrode plate is formed on the top surface of a silicon substrate 42. The diaphragm 43 has a rectangular form, and four corners and midsections of long sides of the diaphragm 43 are supported by anchors 46. A displacement of the diaphragm 43 is minimal on a line D that connects between the anchors 46 at the midsections of the long sides. A displacement maximal point G, at which a displacement is maximal, is present on each side of the line D, and the line D extends in a direction intersecting with a line connecting between the displacement maximal points G.