An electrolysis system with a vertically circulating mercury capillary bundle electrode was developed with a very large electrode area in a minimum space by force feeding mercury and an aqueous solution containing metal ion into a fiber bundle packed densely within a small porous glass tube. In order to test the characteristics and stability of the electrolysis system, the reduction voltammograms of uranyl and ferric ions were measured with changes in the mercury and aqueous flow rates. The aqueous flow rate had a large effect on the electroreaction of the metal ion occurring on the interface between the mercury and the aqueous solution and had to be regulated at an appropriate value for the good development of limiting current shape. The limiting current is linearly proportional to the aqueous flow rate. With a mercury flow rate high enough to keep a capillary continuum of mercury in the fiber bundle, the mercury flow rate had almost no effect on the electroreaction. This developed system was confirmed to be effective and stable enough to control rapidly and continuously the oxidation state of metal ions fed into the system under an appropriate aqueous flow rate, having the property of a high overpotential against the hydrogen evolution reaction.