Abstract: An apparatus for treating a waste solution which contains a nonvolatile substance such as a toner used for development of an electrophotographic film and a liquid which is difficult to volatilize at normal temperatures such as isoparaffin. The waste solution heated from below a multiplicity of vertically elongated guide chambers disposed in a waste solution container rises upward through the guide chambers in the form of vapor and is oxidized by a catalyst. After ignition, even if the heating of the waste solution by a heater is stopped, the waste solution in the waste solution container continues to be heated by oxidation heat.