A fibre-optic Fabry-Perot sensor has a waveguide section (6) forming the optical resonator of an interferometer (4), said section being reflecting at its two end faces (7, 8) and comprising a graded index fibre instead of a monomode fibre. The graded index fibre is preferably bent into a U shape. The transmission light is detected by means of a light detector (11) which is connected to an evaluator circuit (14, 15, 16, 17).