To write an output corresponding to a reference position relative to each liquid level sensor as reference data in a memory of a magnetoelectric device by using a reference position writing tool, and to output a highly-accurate liquid level signal corresponding to the moving position of each float following the reference data.

A Hall IC 20 is provided with the memory for storing an output value of the liquid level, and the uppermost position and the lowermost position of the float 16 set by using the reference position writing tool 21 are written in the memory as the uppermost liquid level and the lowermost liquid level respectively.