PURPOSE: To efficiently decompose difficulty decomposable cyanide compounds such as cyanoferate complex salt, by adding hydrogen peroxide as an oxidizing agent to cyanide waste water, and adding manganese as a catalyst for the oxidizing action of hydrogen peroxide.

CONSTITUTION: Cyanide waste water having cyanide content below 5ppm is introduced into a pretreating vessel 1 to which NaOH or H2SO4 is added, reacted and mixed with manganese as a catalyst. In this case, pH is preferably held above 10 and the addition amount of manganese is preferably controlled above 100ppm. The waste water is then introduced into a decomposing vessel 2, decomposed by reaction with hydrogen peroxide water of concentration above 0.2%, introduced into a reducing vessel 3, and reduced by Na2SO3. Thereafter, the waste water is introduced into a coagulating vessel 4 to which a flocculant is added together with NaOH or H2SO4 and coagulated by coagulating treatment. The resulting supernatant liquid is drained.