Abstract: The flow meter has in a substantially cylindrical inner housing (1) a helically designed diffuser (4), which imparts a swirl to the medium flowing through the inner housing. Downstream of the diffuser there is rotatably mounted a rotor (8), which has blades (13) extending from its rotational spindle (9) as well as a ring (14) which connects the blade ends to one another and is coaxial to the spindle. The rotor (8) is set in rotation by impingement of the blades (13) by the flowing medium. On the outer circumferential surface (15) of the ring (11) there is on a circular line a multiplicity of markings (16) arranged at equal angular intervals from one another. Through a window (18) in the inner housing (1), a light beam strikes the outer circumferential surface (15) of the ring (14), provided with the markings (16). Upon rotation of the rotor (8), the sequence of markings is transmitted by an optical-fibre cable (20) and sensed by a sensor (21), so that from the signals received the flow rate can be determined.