In an area flow meter, float 22 made of material larger in specific gravity than a fluid being measured is movably disposed in vertical pipe 21. Magnet 23 in the float 22 has its north and south poles aligned vertically to produce magnetic-force lines 26 symmetrically with respect to its symmetry axis. Magnetism-gathering tapered element 27 made of a high-permeability material is configured to flare outwardly from its fixed end to its free end to have their fixed ends mounted on magnetism-responsive surfaces of magnetic sensors "A" which are at the same distance from the symmetry axis and disposed outside the pipe on a plane perpendicular to the axis. The flux caught by the elements 27 is gathered on the surfaces to boost only output sensitivity required for detection of the float in position.