A decomposition apparatus for decomposition of decomposition targets such as organic chlorinated compounds is provided. The decomposition apparatus includes a contact section with an internal space to allow a decomposition target to come into contact with and be taken into a liquid, a flow path to lead along with the liquid the decomposition target taken into the liquid to a position different from the internal space, a decomposing device positioned along the flow path in a state cut off from the outside air to decompose the decomposition target that has been led through the flow path, and a device to introduce to the contact section a liquid that contains products produced from the decomposition of the decomposition target by the decomposing device. The decomposing apparatus have plate-shaped electrodes that come into contact with and electrolyze the decomposition target that flows through the flow path, and wherein the surfaces of the electrodes are positioned within the decomposing device at an angle different from the flow direction of the liquid in the flow path, for example, perpendicular to the flow direction of the liquid in the flow path.