A diaphragm-type fiber-optic acoustic sensor is developed. In such a sensor, the sound-induced strain is imposed on the fiber on the diaphragm-type sensing element, and can be measured by the use of a Mach-Zehnder interferometer. The diaphragm structure can provide directional response with high sensitivity. The experimental measurement frequency range is from 100 Hz to 5 kHz. The sensitivity and minimum detectable acoustic pressure of the system are –139 dB re V/μPa and 82 dB re μPa, respectively.