The invention relates to an instrument (1) for measuring the elasticity of a human or animal organ located behind the ribs (2). The inventive instrument is of the type that consists of a casing (3) comprising an actu-ator, the end of which is equipped with a transducer (4) that is actuated in order to measure the elasticity of the organ. The invention is characterised in that the measuring instrument (1) also comprises a means (7) for centring the transducer (4) between the ribs (2).