PURPOSE: To obtain the title measuring device capable of independently and accurately measuring concentration of each ingredient by constituting liquid to be measured introduced into a space to be measured in a measuring vessel so as to apply electric field crossing the above- mentioned measuring space to a definite direction and fractionate the liquid to be measured by fraction means.

CONSTITUTION: In the above-mentioned measuring apparatus, a piston 5 in a cylindrical vessel 6 having measuring space near the inside of the both ends has a passing-through hole 41a passing through the both end and is moved to the both end by application of magnetic field to coil 9. A material to be measured introduced from either one of connecting pipe 2 and 3 into measuring space is fractionated to electric field direction by pairs of electrodes 11a, 11b, 12a and 12b. Then the fractionated material to be measured is moved from either one of measuring spaces through the passing-through hole 41a to other of measuring spaces. After the material to be measured is gradually concentrated to other measuring space by repeating such operation, the concentration is measured. Namely, in the fractionated material to be measured, laser beam is irradiated to the direction crossing with the above-mentioned electric field by luminous means 30 and detected by the first light-receiving means for receiving reflection light or the second light-receiving means for receiving transmission light to carry out concentration measurement.