An ultrasonic flow sensor comprises a flow cell body with an inlet on one side and an outlet on the other side through which the liquid or gaseous media flows; an inner flow conduit with rough surface which not only decreases the flow transition point from turbulence regime to laminar regime but also suppresses the parasitic multi-mode ultrasonic waves; one pair of electro-ultrasonic transducers for transmitting and receiving ultrasound signals; one pair of ultrasonic wave reflecting mirrors for guiding ultrasonic waves from one transducer to the other; and two reflecting mirror support fixtures which set the position and orientation of the ultrasonic wave reflecting mirrors in such a way that their disturbance to the flow inside the inner flow conduit is minimized while the ultrasonic signal strength is maintained.