A method for simultaneously detecting a size and concentration of ionic materials includes measuring voltage drop values of at least three ionic materials of which sizes and concentrations are known using each of at least two FET-based sensors having different electrical characteristics, determining at least three points in a three-dimensional plot from the known sizes, concentrations and the measured voltage drop values, approximating the at least three points into a single plane, measuring a voltage drop value of an ionic material of which size and concentration are unknown using the at least two FET-based sensors, determining equipotential lines existing on the plane using the voltage drop value of the unknown ionic material and determining a cross point between each of the equipotential lines.