Devices for determining at least one parameter of a medium flowing in a conduit, in particular for determining the air-mass flow in the intake tract of an internal combustion engine, including a conduit member and sensor device having a bypass section are already known. To prevent flow detachment at the side walls of the bypass section, it is proposed that a flow guide wall running substantially parallel to the side wall provided with the separation opening of the bypass section be arranged in the conduit member behind the separation opening, as viewed in the main flow direction; the length dimension b of the flow guide wall, as viewed in the main flow direction, being equal to or greater than the distance a of the separation opening from the rear wall; and the distance c of the flow guide wall from the center axis of the conduit member being greater than or equal to the distance d of the side wall provided with the separation opening of the bypass section from the center axis.