An electrochemical machining system using ultrapure water has been developed, designed for planarization machining and free-curvature-shape machining. The machining system is composed of three units: an electrochemical machining apparatus, an ultrapure water refining and circulation unit, and a high-pressure ultrapure water supply unit. The machining apparatus is equipped with an XYθ table and a Zφ machining electrode. A cylindrical or a spherical electrode is used for the machining electrode. The machining chamber is continually filled with refined ultrapure water by the refining and circulation unit. During machining experiments, process by-products, bubbles, and the like around the machining point can be efficiently removed by the high-pressure ultrapure water supply unit. Using this machining system, planarization experiments have been performed on copper plates, which are known to be easily etched, via application of a positive voltage. The effects of various machining parameters (current density, rotating speed of the electrode, etc.) on the roughness of the etched surface are investigated. It was found that the use of a high-pressure ultrapure water supply nozzle is effective in the planarization process. Furthermore, it was also found that there is an ideal set of conditions with respect to the combination of current density and rotating speed of the electrode. Copper plate surface roughness (Ra) was successfully reduced from 164 nm to 10 nm by planarization machining, with the optimal set of parameters.