An unitized reversible cell (URC) which can work as both water electrolyzer and fuel cell is an interesting device in the wide use of hydrogen energy in the near future. At first, we have manufactured a laboratory-scale single cell, for the element technology development, with electrode area of 10 cm2. Next, aiming at high power, we have gradually enlarged the electrode area, as a practical electrode size, to that of 250 cm2. From the results of performance evaluation about those single cells, it was confirmed that there were few difference about both modes performance by the difference of electrode area. Especially, from the results of 250cm2 cell practical investigation, suitable conditions for high performance operation of the URC and the switching criteria of both modes were identified. In addition, it was confirmed that there were no difference between the URC and dedicated systems in the performance dependence on operation condition. Moreover, aiming at further high power, we manufactured the 1 kW-class URC stack which consists of 10 cells, based on the results of single cells. The efficiencies of both water electrolyzer and fuel cell modes of the URC stack were examined. From obtained results, it was confirmed that the energy conversion efficiency of the URC developed here were comparable with that of dedicated systems. In addition, it was experimentally demonstrated that the switching operation mode of the URC between water electrolyzer and fuel cell was performed successfully.