To provide a method for treating waste liquid which decomposes the waste liquid containing chlorine, fluorine, or the like, into the state of safe substances without forming hazardous materials and does not give rise to pollution problems, and a reactor used for the same.

A solid electrolyte layer 1 consisting of, for example, stabilized zirconia is disposed in at least a part of the partition of a container 11 in which liquid is housed. The outer side of the container 11 of the solid electrolyte layer 1 is provided with a first electrode 3 of a porous metal consisting of, for example, platinum. A second electrode 5 for impressing voltage to the waste liquid 2 to be subjected to decomposition or disposal treatment which contains the chlorine and fluorine in the container 11 is disposed in this liquid. A structure in which power sources 8 and 9 are connected between the first electrode 3 and the second electrode 5 is adopted. The chlorine, and the like, in the liquid are subjected to oxidation decomposition treatment by supplying nascent oxygen or ionized oxygen into the liquid.