A device for determining and/or monitoring volume and/or mass flow rate of a medium that flows in a direction of flow through a pipeline/measuring tube of inner diameter. The device comprises at least two ultrasonic transducers that emit and/or receive ultrasonic measuring signals along defined sonic paths and a control/evaluation device which determines the volume flow rate and/or the mass flow rate of the medium to be measured inside said pipeline/measuring tube based on the ultrasonic measuring signals according to the travel-time difference principle. In order to provide a multichannel ultrasonic flowmeter, at least one reflector element is placed in the interior of the pipeline/measuring tube. The reflector element has a defined distance from the inner surface of the pipeline/measuring tube and is placed within the sonic path of the ultrasound measuring signals running through the pipeline/measuring tube.