Devices are previously known for determining at least one parameter of a medium flowing in a conduit, in particular for determining the air mass flow in the intake section of an internal combustion engine, having a conduit part and a sensor device that has a bypass part. In order to avoid a flow detachment at the side walls of the bypass part, it is provided that a flow vane, running at least approximately parallel to a side wall, provided with a separation opening, of the bypass part, be provided in a conduit part behind a separation opening, seen in the main flow direction, and that the conduit part be provided, in the area of the bypass part, with at least one projection that protrudes from an inner wall of the conduit part towards the bypass part.