A Coriolis flow sensor with at least one vibrating flow tube through which a medium flows, includes elements for exciting the tube, and optical detection element for determining the movements of one or several points of the tube based on the principle of reflection of a light beam against the photosensitive surface of a light sensor. The light beam is directed at the tube wall, at a layer provided on this wall, or at an element fastened against this wall, wherein elements for shaping the beam are arranged for making the convergence of the beam in the direction of movement of the tube associated with the Coriolis forces smaller than the convergence in the direction of movement of the tube associated with the excitation forces. The sensitivity in the detection of the small movements of the tube caused by the Coriolis forces is enhanced.