To easily, inexpensively and highly efficiently improve water quality supplied through a lead water supply pipe or the like.

An electrolysis cell 10 is constituted of a porous separator 11 which is tubular and has insulation property and ion conductivity, a counter electrode 15 which is formed at an outer peripheral part of the separator 11, and is immersed in counter electrode solution 17a, and a working electrode 12 arranged at an inner peripheral side of the separator. Then, the working electrode 12 side is made into a positive electrode through a carbon electrode 12a, a prescribed electrical potential is applied between the working electrode 12 and the counter electrode 15, and a liquid to be treated flows in from an inlet 16a. Thereby, when the above liquid to be treated passes along the carbon fiber of the working electrode 12, Pb ions in the liquid to be treated (object to be treated) move in counter electrode solution 17a through each hole (illustration omitted) of the separator 11, the Pb ions which have moved are circulated to a storage tank 20 together with counter electrode solution 17a through piping 19 and a pump 19a, and lead is captured and stored in the storage tank 20.