PURPOSE: To provide a low-cost flowmeter, having high measuring accuracy and reliability, by vertically movably providing a float of specific gravity larger than a fluid to be measured within a vertical passage pipe.

CONSTITUTION: A vertically magnetized magnet 23 is provided in a float 22 or a part vertically moving within a passage pipe 21 according with a flow together with the float 22. At every point on the circumference of the circle at the distance equal from the axis of symmetry on a plane perpendicular to the axis of symmetry, the flux density on the plane perpendicular to a line 26 of magnetic force is the same, and so with an angle between the line 26 and the axis of symmetry, namely the angle p of a tangent of the line 26. The angle of the tangent of the line 26 at the point on the circumference of the circle is held always about constant even if the magnetic force of the magnet 23 is somewhat changed due to temperature change, etc., and the angle of the tangent is determined by the vertical. relative positions of the plane and the magnet 23. Therefore, the position of the magnet 23, namely the float 22 is detected by finding the angle of the tangent of the line 26, and the flow can be measured by the height of the float 22.