The reduction of CO2 emissions to combat global warming is one of the most pressing issues in the electric power business; however, the fact is that no measures are currently available that would effectively reduce electric power transmission and distribution losses, which currently hover around 5%. Likewise, the replacement of large-capacity pipe-type oil-filled cables (POF cables), which are becoming obsolete, is expected to go into full swing sometime around 2016. Replacing them with lowcapacity cross-linked polyethylene vinyl sheath cables (CV cables), however, requires additional circuits, which is rather impractical due to the difficulties inherent in constructing new cable tunnels in urban districts. Because they fit large-capacity, low-loss power transmission hardware into a compact package (1), high-temperature superconducting (HTS) cables are expected to offer innovative solutions to these technical bottlenecks. They have therefore been identified as a key technology for the development of next-generation power transmission grids, and a number of development and demonstration projects are currently underway around the world. Sumitomo Electric Industries, Ltd. has been pursuing the development of HTS wires from the time that HTS materials were first discovered in 1986, and by using HTS wires, it has been working to develop “3-in-One” HTS cables since 1991 (2), (3). Sumitomo Electric became the first in the world to succeed in electricity transmission on a live grid with these cables in the Albany Project in the United States in 2006 (4). Since that time, this long-term demonstration has progressed satisfactorily, unattended for a total of nearly nine months of electricity transmission (5). In the United States, several HTS cable demonstration projects have been conducted in actual power grids; however, no in-grid demonstration is conducted in Japan’s power grids. Because of this, in 2007, the New Energy and Industrial Technology Development Organization (NEDO) commenced the High-temperature Superconducting Cable Demonstration Project, the first operation of HTS cables in a live grid in Japan. Sumitomo Electric has been participating in this project to demonstrate the reliability of HTS cable systems in Japan’s power system. This paper describes the development status of HTS cables in this project and the results of verification conducted prior to the demonstration.