An apparatus for determining at least one parameter of a medium flowing in a main flow direction, in particular of an intake air mass of an internal combustion engine, is proposed. The apparatus has a plug-in part that is introducible into the flowing medium with a predetermined alignment with respect to the main flow direction, having at least one inlet opening and at least one outlet opening as well as at least one main channel connecting the two openings. Also provided is at least one measurement channel that branches off from the main channel, having at least one sensor element, received in the measurement channel, for determining the at least one parameter. At least one directing surface, which diverts the flow of the medium from the measurement channel, is provided before a branching point of the measurement channel from the main channel. The directing surface is disposed at least in part at an angle α different from 90° with respect to a plane that is spanned by the main flow direction and the longitudinal extension axis.