Abstract: An acoustic sensor includes the following: a piezoelectric element constituted of a piezoelectric substance on which at least one electrode is provided, and a diaphragm on which the piezoelectric substance is mounted and supported; a membrane capable of vibrating, which is arranged in a position opposite to the diaphragm; a frame which is so horn-shaped that the interior space is enlarged from the edge of the membrane to the edge of the diaphragm; a cell constituted by arranging the frame in such a way as to house the diaphragm therein, expose the membrane to the outside, and enclose an electrically insulating liquid hermetically therein; and a lead connected to the electrode and drawn out from the electrode to the outside of the frame.