A treatment apparatus for organic waste including a reaction vessel for introducing and decomposing organic waste and pulling out decomposed organic waste as a treated fluid. The reaction vessel includes an internal vessel made of corrosion-resistant material, an external vessel made of pressure-resistant material provided for surrounding the internal vessel via a gap, and an interconnection pipe for interconnecting the gap and the internal vessel outside the external vessel, thereby to control inside the gap and inside the internal vessel to practically equal pressure. The internal vessel has a heating zone for heating the introduced organic waste to a temperature higher than a critical point of water, a reaction zone for holding and decomposing the organic waste at a temperature higher than the critical point of water, and a cooling zone for cooling the treated fluid containing a decomposition product of the organic waste to lower than 100.degree. C. The gap is adapted to be charged with high-pressure water or hydrogen peroxide water.
