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Yes, the warranty will work in Japan. Just for you to know, Intel offers three-year limited warranties on Intel® NUC products purchased from Intel® Authorized Distributors. This warranty covers all hardware items included in the box.
For more information visit:
Intel Customer Support Technician
Under Contract to Intel Corporation
TSone, I can attest that the global warranty does work. My NUC is in Malaysia and I got it replaced there a few weeks ago. Intel finds the way, dont worry. The process is that Intel arranges to get yours sent back to (wherever) and once verified, they send the replacement. The entire process takes about a week, with the arrangements handled in email. Intel is very courteous and responsive about this. No problem.
One more thing which is more important than my first answer. Make the goal to NOT replace the NUC. That is, spend time on the forum to look around and get to know NUCs, especially problems that occurred because of user errors and mistakes. Remember, these are kits and need user intervention to work. As an example, note that the ram you buy should be on the approved list. NUCs dont support 1GB modules, they must be 1.35V, not 1.5V, and if one ram slot is used, use the lower slot. Users come to the forum with problem NUCs and it turned out they used the wrong ram because they didnt take time to first read and learn. Even though I bought my NUC three years ago, I still come and read about problems to increase knowledge. When you get your NUC, come and ask questions and get to know it. There are good people around to help you. There are also tons of NUC videos on youtube to help you install things and get started. You also look up your NUC on Intels main site and download tech sheets and specs and manuals and whatever is there. Look them over. Time spent in learning save much problem later.