An apparatus for determining or monitoring a process variable of a medium in a pipeline, which has a predetermined inner cross section, comprising a sensor and a T-shaped adapter. The adapter has a first portion and a second portion arranged essentially perpendicular to the first portion. The first portion has essentially the same inner cross section as the pipeline and the sensor is so arranged in the second portion that the end face of the sensor facing the medium is flush with the inner surface of the first portion of the adapter.