To increase a tritium concentration in a sample water efficiently to a desired concentration rate by an apparatus having a simple structure.

The multistage type tritium concentrator 1 is a tritium concentrator for increasing a tritium concentration in a sample water by an electrolysis and is constructed by comprising electrolysis cells 3a-3d which have an anode chamber 14 and a cathode chamber 15, an ion-exchange membrane 7 provided near the anode chamber 14 and the cathode chamber 15, and an anode 8 and a cathode 9 respectively provided in the anode chamber 14 and in the cathode chamber 15, wherein a plurality of electrolysis cells 3a-3d are connected in series, and the cathode chamber 15 of the preceding side electrolysis cell and the anode chamber 14 of the following side electrolysis cell are connected by tubes 5b, 5c, and 5d.