The present invention provides a liquid level measuring device which comprises: a tube for accommodating a liquid; a floating member disposed inside the tube, the floating member rising and falling as the level of the liquid varies; at least one magnetic detecting element fastened to the floating member, the magnetic detecting element floating along with the floating member; and a plurality of magnetic sources disposed outside the tube, which are distributed separately along the changing direction of the liquid level so as to establish a specific magnetic field strength distribution, wherein the magnetic detecting element detects the magnetic field strength at the height at which the magnetic detecting element is located and outputs a measuring signal correspondingly, and the liquid level is determined according to the measuring signal. The present invention has many advantages including low cost, efficiency, and it can easily be deployed in considerable amounts.