Abstract: A magnetic treatment device for fluid fuels, which exhibits a cylindrical flow chamber (6) with a conical part (17) widening in the direction of flow (A) and a conical, magnetic insert (16) with at least two flat and/or annular permanent magnets (10,11,12,13) arranged axially at a distance from each other, whereby the magnetic insert is held against the direction of flow with a force, e.g. intrinsic weight and/or the force from a spring and is mounted in the region of the conical part in such a manner that it can be moved axially, and in which the permanent magnets (10,11,12,13) are held at a distance from each other by spacing rings (14) or discs made from non-ferromagnetic material which are placed only in an inner region, between mutually facing surfaces of the permanent magnets, and a bearing shoulder (19) is allocated to at least one permanent magnet (10,12) in the wall of the conical part (17) of the flow-chamber (6).