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 containing at least one sound-producing element and an interfacing shoe and emitting, respectively receiving, ultrasonic measurement signals at an incidence/emergence angle into, respectively out of, the pipeline A first adjusting mechanism is provided, which is so embodied, that the incidence/emergence angle of the ultrasonic sensor is adjustable in predetermined limits. The apparatus further includes a control/evaluation unit, which ascertains the volume- or mass-flow of the medium in the pipeline on the basis of the travel-time difference method, the cross-correlation method or the Doppler method.