Supercritical water oxidation (SCWO) has potential for use in on-site treatment of organic waste because organics can be rapidly decomposed in a closed system. In this study, SCWO of toxic parts of the puffer fish was examined. SCWO of the puffer fish ovary was conducted at 400 degrees C and 25 MPa with an excess amount of oxidant. Tetrodotoxin contained in the ovary was decomposed in 1 min. The small amount of solid residue became constant after 1 min of reaction and was thought to be ash derived from the ovary. In the liquid phase, 4% of total organic carbon (TOC) remained after 10 min of reaction, and a major component was acetic acid. By increasing the reaction temperature to 500 degrees C, remaining TOC decreased to 0.4% after 1 min of reaction. However, ammonia and other N compounds remained in liquid phase after the reaction at 500 degrees C.