The method is for reaction treatment including: an organic waste in which oxygen is difficult to penetrate and in which a biochemical reaction by microbe is difficult to occur. This method includes: forcedly supplying oxygen to inner parts of the organic waste to elevate, by biochemical reactions, the temperature of the inner parts of the organic waste to which oxygen has been supplied; and maintaining the elevated temperature to continue the biochemical reactions and thereby compost the organic waste. Thereafter, the organic waste which has been composted is held in the presence of oxygen and carbon monoxide to cause an exothermic reaction and thereby reduce the volume of the organic waste or carbonize the waste.