PURPOSE: To accomplish a quick measurement of the two-layer electric capacity utilizing a sample holding circuit and arithmetic circuit.

CONSTITUTION: With a sample electrode 2 and an anti-pole 3 immersed in an electrolyte 5 in the measuring electrolytic cell 1, current is fed from a step pulse generator 51 thereto. After the application of the step current, the voltage value is sampled according to the clock signal of the clock signal generator and held with a sample & hold device 53. The voltage value thus held is computed with an arithmetic unit 54 and the result, the two-layer electric capacity, is indicated at a voltage display 55.