PURPOSE: To accurately measure the quantity of a liquid stored in a tank by calculating the quantity of the liquid from a table stored in a memory and level signals received from sensors.

CONSTITUTION: Sensors S1, S2,... and a reference sensor Sref are driven by means of driving units DU1, DU2,... and REF. The outputs of the sensors S1, S2,... and Sref are supplied to a system controlling and processing unit 40 through a mutual conductance amplifier 28 and an A/D converter 30. The unit 20 finds the quantity of a liquid stored in a tank from a table stored in advance in a memory and level signals received from the sensors S1, S2,....