Abstract: The invention relates to a device (2) designed and adapted to bring an in per se known mixing body (11) to correspond to a fluid flow area which e.g. is defined by a pipeline (1,3) wherein a two-phase fluid may flow, said fluid being desired to be mixed in connection with sampling (4), and to withdraw the mixing body (11) easily and rapidly in order to release said fluid flow area. For this purpose, the device according to the invention comprises a valve housing (6) formed for fluid-tight connection to said pipeline in an area of two diametrally opposite openings (1', 1") of the pipeline, enclosing said openings. Within the valve housing (6), a sluice body (5) carrying said mixing body (11) has been displaceably arranged. The sluice body (5) and thus the mixing body (11) is displaceable between two main positions. In one main position, the mixing body (11) has been brought to correspond with said fluid flow area.