PURPOSE: To obtain a high-performance and long-life optical fiber passive acoustic sensor with superior S/N ratio, high sensitivity, and high precision and preferable for measuring the deep sea part on board.

CONSTITUTION: The optical fiber passive acoustic sensor is composed by inserting a spectroscope means for dividing the laser light outputted from a laser light source 1 into a reference laser light B and a modulation laser light A and an acoustic sensor 7 made of single mode optical fibers turned in a coil shape, and provided with a single mode optical fiber 4 for leading modulation laser light, a light route length adjustment unit to make constant the phase difference between a modulation laser light A and a reference laser light B through the acoustic sensor 7, a means for making the modulation laser light A and the reference laser light B interfere with each other, and a signal processing device 6 for taking out electric signals in proportion to the sound pressure from the interference laser light emitted from the interference means.