A new idea of measurement for the liquid-level and the concentration of the electrolytic solution using a single sensor at the same time is presented. A multi-functional sensor with novel structure for achieving this proposed new idea is designed. Two sensing functions, namely, the capacitance and the inductance sensing functions can be realized by this sensor. Unknown concentration and liquid-level of the electrolytic solution in a single tank are estimated in this paper. © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.