Abstract: An acoustic sensor for measuring the linear movement of an internal structure of a nuclear reactor using sound waves, including: an electro-acoustic transducer capable of emitting sound waves; and a waveguide capable of guiding the sound waves emitted by the transducer toward a measurement area of the internal structure, wherein the waveguide is capable of guiding the reflected wave and the waveguide is secured to the measurement area and arranged so as to be capable of extending or retracting in accordance with the movement of the internal structure of the nuclear reactor.