An acoustic sensor comprising: a casing (106, 108) having a pressure hole (106a, 108b) that allows inflow of fluid into the casing (106, 108), wherein the casing houses a MEMS capacitive transducer which comprises: a semiconductor substrate having an opening; a back plate disposed so as to face the opening of the semiconductor substrate, configured to function as a fixed electrode, and having sound holes of circular shape that allow passage of fluid; and a vibration electrode film disposed so as to face the back plate through a void, wherein the acoustic sensor is configured such that deformation of the vibration electrode film is converted into a change in capacitance between the vibration electrode film and the back plate, and a flow path cutoff part is further provided as component of the back plate, the flow path cutoff part being configured to cut off at least a part of a flow path for a fluid that passes through a gap between the vibration electrode film and the semiconductor substrate or the back plate and/or a flow path for a fluid that passes through the opening of the semiconductor substrate or the sound holes of the back plate, when the vibration electrode film is deformed to come close to the semiconductor substrate or the back plate, wherein the flow path cutoff part comprises a plurality of projections (25c, 27c) provided in one of the vibration electrode film and the back plate, and configured to block some of the sound holes in the back plate by abutting with the other one of the vibration electrode film and the back plate when the vibration electrode film is deformed to come close to the back plate, wherein each projection (25c, 27c) in the plurality of projections (25c, 27c) has a cylindrical tube shape such as to enclose a respective one of said some sound holes in the back plate in a plan view. a casing (106, 108) having a pressure hole (106a, 108b) that allows inflow of fluid into the casing (106, 108), wherein the casing houses a MEMS capacitive transducer which comprises: a semiconductor substrate having an opening; a back plate disposed so as to face the opening of the semiconductor substrate, configured to function as a fixed electrode, and having sound holes of circular shape that allow passage of fluid; and a vibration electrode film disposed so as to face the back plate through a void, wherein the acoustic sensor is configured such that deformation of the vibration electrode film is converted into a change in capacitance between the vibration electrode film and the back plate, and a flow path cutoff part is further provided as component of the back plate, the flow path cutoff part being configured to cut off at least a part of a flow path for a fluid that passes through a gap between the vibration electrode film and the semiconductor substrate or the back plate and/or a flow path for a fluid that passes through the opening of the semiconductor substrate or the sound holes of the back plate, when the vibration electrode film is deformed to come close to the semiconductor substrate or the back plate, wherein the flow path cutoff part comprises a plurality of projections (25c, 27c) provided in one of the vibration electrode film and the back plate, and configured to block some of the sound holes in the back plate by abutting with the other one of the vibration electrode film and the back plate when the vibration electrode film is deformed to come close to the back plate, wherein each projection (25c, 27c) in the plurality of projections (25c, 27c) has a cylindrical tube shape such as to enclose a respective one of said some sound holes in the back plate in a plan view. a semiconductor substrate having an opening; a back plate disposed so as to face the opening of the semiconductor substrate, configured to function as a fixed electrode, and having sound holes of circular shape that allow passage of fluid; and a vibration electrode film disposed so as to face the back plate through a void, wherein the acoustic sensor is configured such that deformation of the vibration electrode film is converted into a change in capacitance between the vibration electrode film and the back plate, and a flow path cutoff part is further provided as component of the back plate, the flow path cutoff part being configured to cut off at least a part of a flow path for a fluid that passes through a gap between the vibration electrode film and the semiconductor substrate or the back plate and/or a flow path for a fluid that passes through the opening of the semiconductor substrate or the sound holes of the back plate, when the vibration electrode film is deformed to come close to the semiconductor substrate or the back plate, wherein the flow path cutoff part comprises a plurality of projections (25c, 27c) provided in one of the vibration electrode film and the back plate, and configured to block some of the sound holes in the back plate by abutting with the other one of the vibration electrode film and the back plate when the vibration electrode film is deformed to come close to the back plate, wherein each projection (25c, 27c) in the plurality of projections (25c, 27c) has a cylindrical tube shape such as to enclose a respective one of said some sound holes in the back plate in a plan view.