A continuous-flow reactor system for chemical conversions in supercritical water under superhigh pressure (above 100 MPa) has been developed. An aqueous solution was mixed with preheated water in a swirl-injection mixer. At the mixing point, the solution was rapidly heated to its reaction temperature and the reaction was initiated. The residence time in the tubular reactor was less than 1 s. After passing through the reactor, the solution was quickly cooled with a cooling water jacket to terminate the reaction. Superhigh pressure of the reactor system was controlled at the sampling point using back-pressure regulators. Water-soluble organics (glucose, fructose, etc.) can be converted into useful intermediates and chemicals (glyceraldehyde, 5-hydroxymethylfurfural, lactic acid, etc.) in subcritical and supercritical water at 523-873 K, 20-200 MPa using this system. As a preliminary study of conversions under supercritical pressure, the fluid behavior in the mixer was investigated using a three-dimensional simulation based on the finite volume method.