An apparatus for ascertaining, and/or monitoring, volume-or mass-flow of a medium flowing through a pipeline in the direction of the longitudinal axis of the pipeline. The apparatus includes: at least one ultrasonic sensor, which radiates/receives ultrasonic measuring signals at an angle of incidence/emergence into/out-of the pipeline; and a control/evaluation unit which provides, based on the ultrasonic measuring signals, information concerning volume- and/or mass-flow of the medium through the pipeline. The ultrasonic sensor includes at least one sound-producing element, a coupling shoe and a coupling adapter. The end region of the coupling adapter facing the coupling shoe has a securement mechanism for releasable securement of the coupling adapter to the coupling shoe. The end region of the coupling adapter facing the pipeline is embodied as a function of the pipeline in such a manner that it is form-fittingly securable to the pipeline.