To make chemical agent addition unnecessary or to lower the addition amount of a chemical agent by carrying out electrolytic treatment in the presence of a polyvalent metal ion by employing an electrolytic bath containing a partition made of a cation exchange membrane and introducing fluorine- containing water to the cathode chamber of the electrolytic bath.

Electrolytic treatment of fluorine-containing water is carried out in the presence of a polyvalent metal ion by employing an membrane electrolytic bath 1 partitioned with a membrane 2 of a cation exchange membrane into an anode chamber 3 and a cathode chamber 4 and introducing fluorine- containing water as raw water to the cathode chamber of the electrolytic bath 1. By this method, metal hydroxides are precipitated by OH ion generated in the cathode side by water electrolysis and together with the metal hydroxides, fluorine is co-precipitated and removed. Aluminum ion (Al3+), magnesium ion (Mg2+), and the likes are examples of the polyvalent metal ion and since hydroxides can be precipitated in the case alkalinity is pH 9 or higher and no fine pH adjustment is required, Mg2+ is preferable, Introduction of raw water to the cathode chamber 4 is preferably carried out by circulating raw water.