Provided is an acoustic sensor capable of improving an S/N ratio of a sensor without preventing reduction in size of the sensor. A back chamber 45 is vertically opened in a silicon substrate 42. A thin film-like diaphragm 43 to serve as a movable electrode plate is formed on the top surface of the substrate 42 so as to cover the back chamber 45. The back plate 48 is fixed to the top surface of the substrate 42 so as to cover the diaphragm 43, and a fixed electrode plate 49 is provided on the under surface of the back plate 48. Further, the diaphragm 43 is divided into a plurality of areas by the slit 47, and the respective plurally divided diaphragms 43a, 43b and the fixed electrode plate 49 constitute a plurality of parallelly connected capacitors (acoustic sensing sections 60a, 60b).