Seems that NRT has luggage storage
Has anyone ever used?

k

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Yes, there is a manned booth at the arrival level of both terminals, in a far corner.

I never felt so safe leaving my luggage somewhere... it was not inexpensive however.

Japanese lockers are not suitable for extended storage, as they 'time-out' after a day, IIRC, requiring human intervention to get them re-opened.

It is indeed very safe to use luggage storage at Narita. Japanese are usually very responsible for such kind of service.
You do not need to worry about losing your luggage.
I, my wife, my sister-in-law, and my frends all used their service and we had no problem.
I remember it costs only $4-$5 per piece a day.