To provide a clamp-on type ultrasonic flow sensor capable of preventing deterioration in measurement accuracy of a flow rate due to deformation of a pipe. Elastic couplants CP1 and CP2 formed to surround the outer circumferential surface of a pipe are fixed while being pressed on the pipe P by fixing inner surfaces FS of two clamp members 131 and 132. An ultrasonic wave is propagated through the elastic couplants CP1 and CP2 and fluid in the pipe between a first ultrasonic element 101 and a second ultrasonic element 102. A cross section of the fixing inner surfaces FS of the clamp members 131 and 132 has a shape of a regular even-numbered polygonal shape centering on the axis of the pipe.